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The 
Root  Habits  of  Desert  Plants 


BY 


WILLIAM  AUSTIN  CANNON 


WASHINGTON,  D.  C. 

PUBUSHEU    BY    THE   CaRNEGIE    INSTITUTION    OF   WASHINGTON 
I9II 


®lje  ^.  p.  lltU  plirarg 


CIL 


The 
Root  Habits  of  Desert  Plants 


BY 


WILLIAM  AUSTIN  CANNON 


WASHINGTON,   D.  C. 

Published  by  the  Carnegie  Institution'  of  Washington 

191 1 


^ 


^V 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
Publication  No.  131 


Copies  0^  this  Book 
were  first  issued 

MAR  28  1911 


PRESS    OF    GIBSON    BROS. 
WASHIXGTON,  D.  C. 


•  tl»HM/ff 


CONTENTS 


Introduction 7 

Certain  Features  in  the  Attempted  Classification  of  Roots 9 

Scope  of  the  Study  and  Methods 10 

Environment  of  Roots 12 

Leading  Characteristics  of  the  Soil 12 

Tumamoc  Hill 12 

Flood-plain  of  the  Santa  Cruz  river 12 

West  Wash 13 

The  Bajada 13 

Moisture  in  the  Soil 15 

Tumamoc  Hill 15 

The  Bajada 17 

The  West  Wash 18 

Flood-plain  of  the  Santa  Cruz  river 19 

Temperature  of  the  Soil 20 

Record  of  Thermograph:   15  cm.  depth 20 

Record  of  Thermograph:  30  cm.  depth 21 

Soil  Temperatures:  2.5  cm.  depth 22 

Summary  of  Environmental  Conditions 23 

Root  Habits  of  Desert  Plants 25 

Summer  Annuals 25 

Amaranthus  palmeri , .  25 

Anoda  thurberi 26 

Aster  tanacetifolius 26 

Boerhaavia  sp 26 

Cladothrix  lanuginosa 27 

Datura  sp 27 

Ditaxis  humilis 27 

Dysoda  papposa 28 

Euphorbia  glyptosperma 28 

Kallstroemia  grandiflora 28 

Pectis  prostrata 29 

Solanum  ekeagnifolium 29 

Trianthema  portulacastrum 29 

Vicia  sp 29 

Winter  Annuals : 30 

Amsinckia  spectabilis 30 

Astragalus  nuttallianus 31 

Bowlesia  lobata 31 

Brodia^a  capitata 31 

Daucus  pusillus 33 

Eritrichium  pterocaryum  and  Harpagonella  palmeri 33 

Erodium  cicutarium 34 

Festuca  octoflora  and  Hordeum  murinum 34 

Gilia  bigelowii 34 

Malva  borealis 35 

Medicago  denticulata 35 

Mentzelia  albicaulis 3^ 

Microseris  linearifolia 3^ 


4  CONTENTS. 

Root  Habits  of  Desert  Plants — Continued. 

Winter  Annuals — Continued.  page. 

Monolepis  chenopodioides 36 

Orthocarpus  purpurascens 36 

Parietaria  delailis 38 

Phacelia  tanacetifolia 38 

Plantago  fastigiata 38 

Rafinesquia  neo-mexicana 39 

Streptanthus  arizonicus,  Sisymbrium  canescens,  Sisymbrium  reflexum  .  39 

General  Conclusions  and  Summary  of  Studies  on  Annuals 40 

Root  Systems  of  Perennials 43 

Plants  from  Tumamoc  Hill 44 

Echinocactus  wislizeni 44 

Encelia  farinosa 46 

Opuntia  discata 48 

Opuntia  leptocaulis 50 

Opuntia  versicolor 52 

The  Bajada 55 

Carnegiea  gigantea 55 

Covillea  tridentata 58 

Fouquieria  splendens 61 

Franseria  deltoidea 64 

Krameria  canescens 67 

Opuntia  arbuscula 69 

Opuntia  fulgida 71 

Riddellia  cooperi 73 

Dasylirion  texanum,  Yucca  radiosa,  Yucca  sp.,  and  Agave  sp 75 

The  Flood-plain 78 

The  Flood-plain  of  the  Santa  Cruz 78 

Koerberlinia  spinosa 78 

Prosopis  velutina 80 

Condalia  spathulata  and  Zizyphus  parryi 81 

Ephedra  trifurca 82 

Opuntia  vivipara 83 

Experimental  Cultures 84 

Physiological  Features  of  Root-Systems 87 

"Character"  in  Roots 87 

The  Relation  of  Roots  to  Soil  Temperature 88 

The  Relation  of  Roots  to  Water 89 

Relations  of  the  Roots  of  Neighboring  Plants 92 

Root  Habits  and  Plant  Distribution 94 

Summary 95 


LIST  OF  PLATES. 


Pacing 
Plate  I.  Pagb. 

A.  Upper  soil,  adobe  clay,  of  Santa  Cruz  flood-plain,  at  place  shown  over  4  m. 

in  thickness. 

B.  Adobe  clay,  the  darker  soil,  overlying  the  caliche  hardpan,  with  "rotten" 

caliche,  broken  masses  of  the  hardpan,  between 1 

Plate  2. 

(o)  Amaranthus  palmeri,  Tumamoc  Hill,  (b)  Anoda  thurberi,  Tumamoc 
Hill,  (c)  Aster  tanacetifolius,  Tumamoc  Hill,  (d)  Aster  tan- 
acetifolius  showing,  at  the  left,  root-formation  at  the  base  of 
lateral  of  first  order 

Plate  3- 

Root  habits  of  summer  annuals,  Tumamoc  Hill,    (a)  Boerhaavia  sp.    (b)  Cla- 

dothrix  lanuginosa,    (c)  Datura  sp.     (d)  Ditaxis  humilis 2{ 

Plate  4. 

Root  habits  of  summer  annuals,  August,  1909.  (a)  Dysoda  papposa,  flood- 
plain  of  the  Santa  Cruz,  (b)  Euphorbia  glyptosperma,  Tumamoc 
Hill,  (c)  Kallstroemia  grandiflora,  Tumamoc  Hill,  (d)  Pectis 
prostrata,  Tumamoc  Hill,  (e)  Solanum  elseagnifolium,  Tumamoc 
Hill 2g 

Plate  5. 

A.  Root  habits  of  summer  annuals,  (a)  and  (b),  Aug.,  1909.     (a)  Trianthema 

portulacastrum,  flood-plain  of  Santa  Cruz,     (b)  Vicia  sp.,  Tuma- 
moc Hill. 

B.  Root  habits  of  winter  annuals,  (c)  and  (d),  March,  1906  and  1907.     (c) 

Amsinckia   spectabilis,    Tumamoc   Hill,      (d)    Daucus   pusillus, 

Tumamoc  Hill 32 

Plate  6. 

Root  habits  of  winter  annuals,  Tumamoc  Hill,     (a)  Festuca  octoflora.     (b) 
Gilia  bigelowii.    (c)  Malva  borealis,  showing  development  of  root- 
system,    (d)  Medicago  denticulata.    (e)  Microseris  linearifolia. ...  32 
Plate  7. 

Root  habits  of  winter  annuals,  Tumamoc  Hill,  (a)  Monolepis  chenopodioides. 
(b)  Orthocarpus  purpurascens  parasitic  on  Astragalus  sp.  (c)  Or- 
thocarpus  purpurascens  on  Lupinus  sp.  (d)  Parietaria  debilis, 
rnature  plants  from  favorable  and  unfavorable  moisture  condi- 
tions,   (e)  Phacelia  tanacetifoHa.    (/)  Plantago  fastigiata 36 

Plate  8. 

Root  habits  of  winter  annuals,  Tumamoc  Hill,  March,  1907.    (a)  Sisymbrium 

canescens.    (b)  Streptanthus  californicus 36 

Plate  9. 

Root-system  of  Echinocactus  wislizeni.  (a)  Portion  of  system  showing  an- 
choring and  absorbing  roots  in  natural  position,  from  West  Wash. 

(b)  Bird's-eye  view  of  roots  from  which  overlying  soil  has  been 
removed,  Tumamoc  Hill 44 

Plate  10. 

(a)  Root  and  shoot  habit  of  a  specimen  of  Opuntia  leptocaulis  growing  under 
the  protection  of  Acacia  constricta,  Tumamoc  Hill,  (b)  Optunia 
versicolor,  showing  habit  of  plant  and  character  of  environment. 

(c)  Bird's-eye  view  of  a  portion  of  the  centra!  part  of  the  root-sys- 
tem of  the  plant  shown  in  b.  (d)  This,  and  figures  on  plate  1 1,  are 
of  one  main  lateral  and  some  of  its  branches  of  the  plant  shown 

above 44 

Plate  h. 

Plate  10,  continued;  bird's-eye  view  of  root-system  of  Opuntia  versicolor. . .  44 

Plate  12. 

(a)  Vertical  and  {b)  horizontal  extensions  of  root-system  of  Opuntia  versi- 
color, views  of  which  are  shown  in  plates  lo  and  11,  Tumamoc 
Hill,  1907 52 


24 


6  LIST   OF   PLATES. 

Facing 
Page. 

Plate  13- 

Root  habit  of  Jatropha  cardiophylla  of  which  the  horizontal  extensions  of  the 

root-system  were  shown  in  plate  12 52 

Plate  14. 

Horizontal  and  vertical  extensions  of  root-systems  of  Carnegiea  gigantea, 

Covillea  tridentata,  and  Parkinsonia  microphylla 56 

Plate  15- 

(a)  The  anchoring  roots  and  bases  of  some  lateral  roots  of  Carnegiea  gigantea 
6.8  m.  high,  (b)  Tap  root  and  secondary  roots  making  up  the 
anchoring  system,  and  the  bases  of  some  of  the  superficial  roots 
of  a  specimen  of  Carnegiea  1.2  m.  high  from  same  habitat  as  cactus 
shown  in  (a),  (c)  Tap  root  and  bases  of  laterals  of  Parkinsonia 
microphylla  growing  near  Carnegiea  shown  in   (b)   and  which 

appears  in  plate  14 56 

Plate  16. 

(a)  Covillea  tridentata.  Horizontal  and  vertical  extensions  of  roots  of  this 
plant  are  shown  in  fig.  8.  (b)  Covillea  from  flood-plain  near  West 
Wash,  showing  the  large  number  of  slender  adventitious  roots 
springing  from  upper  portion  of  the  main  roots  and  long  tap  root 
of  which  only  a  part  appears,  (c)  Fouquieria  splendens  from 
bajada  where  Covillea  shown  in  (a)  was  growing.     Horizontal 

extension  of  roots  of  this  plant  shown  in  fig.  8 64 

Plate  17. 

(a)  Franseria  deltoidea  from  flood-plain  near  West  Wash,  showing  part 
of  tap  root,  (b)  Lateral  of  the  first  order  bearing  filamentous 
roots  in  groups,  which,  at  time  photograph  was  made,  were  no 
longer  functional,  (c)  Krameria  canescens  from  the  flood-plain 
near  West  Wash,  showing  essential  superficial  placing  of  roots. 

(d)  Lycium  andersonii  from  near  West  Wash 64 

Plate  18. 

Bird's-eye  view  of  superficial  roots  of  Opuntia  arbuscula  from  bajada  a  mile 

east  of  Santa  Cruz,  showing  their  fleshy  character 68 

Plate  19. 

(a)  Riddellia  cooperi  showing  the  large  number  of  slender  laterals  arising 
from  crown  of  tap  root,  and  the  branching  feature  of  the  bases  of 
the  laterals,  (b)  Riddellia  from  red  clay  soil,  to  iflustrate  lack  of 
slender  roots  at  crown  of  tap  root,  (c)  Dasylirion  texanum,  with 
root-system  partly  exposed,     (d)  Shoot  and  root  habit  of  Yucca 

sp.     (e)  Root  character  of  Yucca  radiosa 72 

Plate  20. 

Root  habit  of  Kceberlinia  spinosa.  (a)  Isolated  plant  with  prom.inent  tap  root. 
(b)  Several  shoots  arising  from  a  single  sucker  which  still  keeps  its 
connection  with  parent  tap  root,  (c)  Adventitious  absorption 
roots  arising  from  a  sucker  which  has  not  formed  an  independent 
tap  root,     (d)  Secondary  formation  of  tap  root  in  a  group  with 

connecting  sucker  root  no  longer  living 80 

Plate  21. 

(a)  Peniocereus  greggii  showing  fleshy  main  root  and  the  most  important 
laterals,  (b)  Condalia  spathulata.  (c)  Root-system  of  Prosopis 
velutina  partly  exposed  by  the  caving  bank  of  the  Santa  Cruz. 
The  vertical  distance  from  the  ground  surface  to  the  water  is 
approximately  5  m.  (d)  Young  plant  of  Prosopis.  The  squares 
are  centimeters,  (e,  f,  g)  Cuttings  of  Opuntia  arbuscula  (Tucson), 
O.  arbuscula  (Sacaton),  and  O.  vivipara,  respectively,  all  grown 

under  similar  conditions  and  with  a  large  water  supply 80 

.Plate  22. 

(a)  Cultures  of  Fouquieria  splendens  and  of  garden  water-melon  showing 
differences  in  character  and  development  of  extreme  types  of 
root-systems,     (b)  Shoot  of  Opuntia  vivipara  springing  from  a 

fleshy  root 84 

Plate  23. 

Brodiaea  capitata  grown  in  adobe  clay  (a)  and  in  sand  (b)  in  plant-house  cul- 
tures. Main  root  of  clay-grown  plant  curved  away  from  source 
of  water  supply 88 


The  Root  Habits  of  Desert  Plants. 


INTRODUCTION. 

It  is  now  generally  recognized  that  the  arid  and  semi-arid  regions  are 
especially  favorable  areas  in  which  to  study  the  habits  of  plants.  This  is 
partly  because  the  vegetation  of  the  less  humid  countries  represents  the 
most  advanced  type  of  land  forms — that  is,  those  farthest  removed  from 
the  primitive  water-loving  plants — and  partly  because,  as  the  environ- 
mental features  are  severe,  so  the  response  on  the  part  of  the  plants  is  corre- 
spondingly effective  in  order  to  bring  about  survival.  The  shoot  habits  of 
desert  plants  have  received  considerable  attention  from  botanists,  but  the 
reaction  of  roots  to  desert  conditions  has,  in  a  large  measure,  been  neglected. 
That  this  neglect  is  illogical  and  without  good  reason  is  apparent  when  it 
is  recalled  that  the  connection  between  the  plant  and  a  most  important 
feature  of  the  environment,  the  soil  and  its  water  content,  is  sustained 
only  through  the  roots. 

The  fact  is  well  known  that  the  roots  of  plants  have  a  twofold  function. 
They  at  once  afford  safe  anchorage  and  support  and  at  the  same  time  are 
the  means  by  which  water  and  inorganic  food  materials  are  acquired.  The 
dual  nature  of  roots  does  not  find  separate  organic  expression  in  the  most 
primitive  plants,  and  in  certain  of  them,  the  algae,  organs  answering  to 
roots  serve  the  purpose  of  anchorage  only.  In  the  low  land  plants,  the 
roots,  morphologically  rhizoids,  are,  from  this  point  of  view,  quite  undif- 
ferentiated, but  as  the  scale  of  plant  life  is  ascended  we  find  these  functions 
separated  both  in  time  and  in  space,  at  first  on  the  same  root  and  its 
immediate  branches,  and  finally  there  is  differentiation  in  the  root-system 
of  the  plant,  by  which  the  anchorage  is  largely  taken  over  by  one  set  of 
roots  and  the  absorption  by  quite  another  set.  The  last  is  the  advanced 
condition  found  in  the  extreme  xerophytic  desert  plants,  such  as  many  of 
the  cacti.  Thus  the  extent  and  the  character  of  root  development  will 
reveal,  in  a  measure,  the  degree  of  xerophily  of  a  plant. 

The  prevailing  idea  that  the  roots  of  plants  of  the  deserts,  or  of  semi- 
arid  regions,  are  of  great  length,  especially  that  they  penetrate  the  ground 
to  great  depths,  doubtless  has  its  origin  largely  in  the  belief  that  desert 
plants  are  obliged  to  develop  a  deeply  placed  root-system  in  order  to  obtain 
water  during  long  dry  seasons,  and  also  in  the  few  striking  examples  of 
really  long  roots  of  plants  of  arid  countries  which  are  accented  as  repre- 
senting the  root  condition  of  all  desert  plants. 

7 


8  ROOT  HABITS  OF  DESERT  PLANTS. 

On  this  point  Schimper  (Plant-geography,  Bnghsh  edition,  p.  612)  says: 

This  second  category  of  desert  plants  exhibits  its  dependence  on'subterranean  water 
nearly  universally  by  the  immense  length  of  its  root-system,  which  the  depth  of  the 
level  of  the  subterranean  water  renders  vitally  necessary.  A  considerable  length  of  root 
is,  to  a  more  or  less  extent,  common  to  all  desert  plants  and  has  attracted  the  notice 
of  all  travelers. 

This  statement  is  substantiated  by  a  quotation  from  Volkens,  who  says 
(Die  Flora  der  aegyptisch-arabischen  Wiiste  auf  Grundlage  anatomisch- 
physiologischer  Forschungen,  p.  7) : 

Often  as  I  have  tried  to  dig  up  old  bushes  of  perennial  plants  to  the  extremity  of 
their  roots,  I  have  never  succeeded  in  doing  so.  The  most  that  I  could  establish  was, 
that  the  root  was  thinner  at  the  depth  of  one  or  two  meters  than  at  the  surface  of  the 
ground.  One  can  safely  assume  that,  in  this  case,  the  length  of  the  subterranean  part 
was  at  least  twenty  times  that  of  the  epigeous  part. 

Volkens  then  goes  on  to  say  that  certain  species  of  Acacia  were  said  to 
have  been  seen  at  the  time  of  the  digging  of  the  Suez  canal,  whose  roots 
were  found  in  its  bed,  although  the  trees  to  which  they  belonged  were 
growing  on  eminences  on  its  banks. 

Isolated  observations  indicate  that  the  roots  of  certain  trees  of  the  arid 
regions  of  the  southwestern  part  of  the  United  States  may,  under  favorable 
conditions,  form  long  or  deeply  penetrating  roots.  Thus  Prof.  R.  H. 
Forbes,  director  of  the  Arizona  Experiment  Station,  informs  me  that  he 
has  seen  roots  of  the  mesquite  (Prosopis  velutina),  by  an  irrigating  ditch, 
extend  very  near  the  surface  of  the  ground  as  far  as  15  meters,  and  roots 
of  the  same  species,  where  exposed  by  the  washing  away  of  river  banks, 
which  penetrated  as  deep  as  8  meters.  These  figures  may  be  taken  as 
probably  representing  the  deepest  root  penetration  in  this  vicinity,  although 
it  is  no  unusual  thing  to  see  mesquite  roots  5  meters  in  length.  Dr.  V. 
Havard  is  quoted  as  writing  that  sometimes  in  the  Southwest  camps  were 
pitched  on  the  plains  where  there  was  no  fuel  of  any  sort  to  be  seen.  It 
is  there  that  the  frontiersman,  armed  with  a  spade,  went  digging  for  wood. 
Speaking  of  the  deep-penetrating  roots  of  mesquite  Havard  says : 

Of  the  vertical  roots,  the  tap  root  is  the  only  long  and  conspicuous  one.  It  plunges 
down  to  a  prodigious  depth,  varying  with  that  at  which  moisture  is  obtainable.  On 
the  side  of  gulches  one  can  trace  these  roots  down  thirty  or  forty  feet.  (American 
Naturalist,  vol.  18,  p.  451,  1884.) 

It  will  appear  from  facts  given  in  this  paper  that  the  root-systems  of 
different  perennials  growing  in  the  vicinity  of  the  Desert  Taboratorv  are 
extremely  variable  as  regards  depth  of  penetration,  lateral  extent,  and 
other  characteristics,  and  that  no  one  type  of  root  can  be  said  to  be  the 
prevalent  one. 

The  opposite  extreme  in  the  position  of  the  roots  of  desert  perennials  is 
to  be  found  in  a  highly  specialized  class,  the  succulents,  in  which  the  roots 
are  uniformly  near  the  surface  of  the  ground.  Volkens,  for  example  (Die 
Flora  der  aegyptisch-arabischen  Wiiste,  p.  24),  states  that  Enphorbia  has 


INTRODUCTION.  9 

laterals  which  are  superficially  placed,  and  Weiss  and  Yapp  (Sketches  of 
vegetation  at  home  and  abroad,  III.  "The  Karoo"  in  August.  The  New 
Phytologist,  vol.  v.  May  and  June,  1906)  have  reported  the  formation  of 
superficial  in  addition  to  deep-growing  roots  in  Mesembrianthemiim.  In  a 
brief  account  of  the  habits  of  several  cacti  of  Arizona  Preston  also  describes 
the  shallow  placing  of  the  roots  of  several  of  them  (Botanical  Gazette,  vol. 
30,  p.  348,  1900). 

The  dwarfing  of  the  shoots  of  the  perennials  of  the  deserts,  resulting  from 
excessive  evaporation  and  inadequate  water  supply,  does  not  indicate  the 
best  conditions  for  maximum  root  development.  It  is  probable,  on  the 
contrary,  that  the  longest  or  the  most  deeply  penetrating  roots  are  found 
where  there  is  considerable  rainfall  and  where  the  penetration  of  the  rain 
is  considerable  and  the  water  table  relatively  deep.  In  California,  under 
field  conditions,  the  roots  of  the  grape  may  reach  a  depth  of  22  feet  (Hilgard : 
Soils,  p.  167),  and  in  Nebraska  the  roots  of  Shcpherdia  are  said  to  attain  a 
depth  of  50  feet  (Merrill:  Rocks  and  Rockweathering,  p.  181). 

CERTAIN  FEATURES  IN  THE  ATTEMPTED  CLASSIFICATION  OF  ROOTS. 

From  the  point  of  view  of  this  study  any  classification  of  root-systems 
which  has  for  its  final  aim  a  better  understanding  of  the  habits  of  plants 
should  receive  attention,  while  those  classifications  which  are  purely  sys- 
tematic, although  of  value  in  other  lines  of  research,  may  in  this  place  be 
neglected.  Attention  will  be  called,  therefore,  merely  to  the  work  of  Rim- 
bach,  Biisgen,  and  Freidenfeldt,  mainly  as  reviewed  by  von  Alten  (Wurzel- 
studien,  Bot.  Zeit.,  vol.  67,  1909,  p.  175),  which  is  to  be  largely  interpreted 
in  physiological  terms. 

The  researches  of  the  authors  referred  to  indicate  that  the  root-systems 
of  flowering  plants  may  be  separated  into  two  groups  according  to  the 
character  of  the  terminal  roots — they  are  either  intensive  or  extensive. 
Intensive  root-systems  are  such  as  have  fine  terminal  roots ;  they  are  richly 
branched  and  occupy  a  relatively  small  soil  volume.  Extensive  root- 
systems,  on  the  other  hand,  are  such  as  have  coarse  ultimate  rootlets,  are 
not  richly  branched,  and  occupy  a  relatively  large  soil  volume.  An  im- 
portant additional  distinction,  advanced  by  Freidenfeldt,  is  that  of  the 
diameter-quotient,  or  the  relative  diameter  of  the  central  cylinder  of  such 
rootlets  to  that  of  the  entire  rootlet.  Von  Alten  states  that  the  diameter- 
quotient  in  intensive  root-systems  is  greater  than  in  extensive  root-systems. 
For  example,  the  difference  may  be  from  one-third  or  one-fourth  to  one- 
fifth  or  one-sixth,  respectively,  for  the  two  types.  In  addition,  there  are 
structural  differences  which  need  not  be  given  in  this  place. 

Freidenfeldt,  in  place  of  the  term  intensive,  classifies  plants  having  these 
root  types  as  xerophytes,  and  in  place  of  the  term  extensive,  classifies 
plants  with  this  type  of  root-system  as  hydrophytes.  The  beech  is  given 
as  an  example  of  the  former  and  the  ash  as  an  example  of  the  latter,  but 


lO  ROOT   HABITS    OF    DESERT    PLANTS. 

the  terms  xerophyte  and  hydrophyte  denote  more  than  intensive  and  exten- 
sive, as  von  Alten  points  out,  and  from  a  physiological  standpoint,  the 
latter  terms  are  more  useful. 

How  susceptible  to  modification,  through  a  variation  in  the  soil  medium 
or  its  water  content,  the  two  classes  of  roots  may  be,  has  not  yet  been 
determined.  It  is  believed,  however,  that  the  conservative  inheritance 
tendencies  are  more  powerful  here  than  the  influence  of  changing  environ- 
ment, and  that  the  types  will  remain  essentially  constant  under  whatever 
condition  they  may  be  found. 

Among  plants  with  intensive  form  of  root-system  are :  Mentha  piperita, 
Digitalis  purpurea,  Artemisia  vulgaris, Imperatoria  ostruthium.     Plants  hav- 
ing the  extensive  form  of  root-systems  include :  Helleborus  purpurascens, 
Solidago  canadensis,  'Poly soma  integrifolium,  Valeriana  officinalis.  Arnica 
chaniisonis,  Ranunculus  acer,  Adonis  vernalis,  and  others. 

SCOPE  OF  THE  STUDY  AND  METHODS. 

When  it  was  proposed  some  time  ago  to  take  up  the  study  of  the  roots  of 
desert  plants,  the  work  was  laid  down  on  broad  physiological-ecological 
grounds,  and  it  was  realized  that  as  an  introduction  to  such  studies,  which 
would  be  in  the  nature  of  things  mainly  experimental,  a  knowledge  of  the 
habits  of  annuals  and  of  perennials  afield  was  a  prerequisite.  As  an  exact 
description  of  the  root-systems  of  the  most  characteristic  forms  was  wanting, 
the  prosaic  task  of  excavating  roots  was  undertaken,  and  the  present  paper 
includes  an  account  of  the  results  of  this  work. 

While  the  plants  selected  for  study  have  been  such  as  are  presumably 
representative  of  all  types,  they  do  not  include  many  which  would  be  of 
interest,  but  an  account  of  which  would  in  a  degree  duplicate  results  already 
attained.  Aside  from  the  usefulness  of  the  bare  descriptions  of  root- 
systems  in  later  experimental  work,  they  will  probably  be  found  of  value 
in  comparative  root  studies,  where  the  root-systems  of  other  arid  regions, 
in  which  the  physical  conditions  are  otherwise  different  from  those  of  the 
Southwest,  are  carefully  worked  up  and  mapped.  Much  interest  has  been 
found  in  comparing  the  nature  of  the  root-systems  and  their  variability  as 
observed  in  the  field  with  the  leading  and  obvious  characters  of  the  environ- 
ment, and  it  has  been  found,  as  will  be  related  later,  that  in  certain  instances 
there  is  a  very  clear  relation  between  root  type  and  plant  distribution,  as 
well  as  between  root  type  and  other  habits  of  the  plant.  The  root-systems, 
therefore,  have  been  studied  only  in  the  field,  and  natural  conditions  h,  ve 
uniformly  been  described. 

In  the  course  of  the  research  different  methods  have  been  developed  to 
suit  the  particular  form  of  roots  studied.  In  the  case  of  annuals,  the  entire 
root-S3'stem,  or  as  much  of  it  as  could  be  removed  from  the  ground,  was 
measured  and  photographed,  the  photographs  being  somewhat  under  life 
size. 


INTRODUCTION.  I  i 

The  root-systems  of  perennials  had  to  be  studied  in  another  way  and 
always  in  situ.  After  the  earth  above  the  most  important  roots  was 
removed,  the  root-area  of  the  plant  was  considered  as  a  square  and  was 
surveyed  in  the  following  manner.  Tapes  divided  according  to  the  metric 
system  were  stretched  along  the  east  and  west  sides  of  the  imaginary  square, 
in  a  north-and-south  direction,  always  at  a  certain  distance,  1.5  meters 
from  the  base  of  the  stem.  These  tapes  were  firmly  fastened  to  the  ground. 
A  third  movable  tape  was  extended  in  an  easterly  and  westerly  direction 
so  that  it  connected  the  two  permanent  tapes.  This  was  also  ruled  metri- 
cally. By  means  of  moving  the  latter  tape  and  noting  its  position  on  the 
stationary  tapes,  the  north-and-south  as  well  as  the  east-and-west  exten- 
sions of  the  roots  were  learned  with  fair  accuracy.  The  record  was  made 
on  a  square  of  metrically  ruled  paper,  each  square  on  the  paper  correspond- 
ing with  an  imaginary  square  of  the  root-area  as  delimited  by  the  divisions 
of  the  tapes.  Wherever  possible  a  reduction  to  one-tenth  was  employed  in 
sketching  the  roots  and  preparing  the  field  charts. 

The  data  on  the  temperature  of  the  air  and  soil,  on  water  in  the  soil,  and 
on  its  physical  nature  are  taken  from  records  made  at  the  Desert  Labora- 
tory or  from  studies  made  elsewhere  but  now  available  at  the  Laboratory. 
The  soil  temperatures  are  mostly  from  the  continuous  soil-thermograph 
record  which  has  been  kept  since  1904-05,  and  partly  from  thermometer 
readings  made  by  Dr.  V.  M.  Spalding.  The  other  data  on  the  soil  are 
mainly  from  studies  by  Dr.  B.  E.  Livingston  (Distribution  and  Movements 
of  Desert  Plants,  Carnegie  Institution  of  Washington,  Publication  113,  pp. 
83-93)-  Acknowledgment  should  also  be  made  to  Prof.  J.  J.  Thornber, 
University  of  Arizona,  for  the  determination  of  several  of  the  plants  studied, 
as  well  as  for  much  information  on  the  distribution  of  the  plants  of  the 
domain  of  the  Desert  Laboratory  (Vegetation  Groups  of  the  Desert  Labora- 
tory Domain,  Carnegie  Institution  of  Washington,  Pub.  113,  pp.  103-112). 


ENVIRONMENT  OF  THE  ROOTS. 

LEADING  CHARACTERISTICS  OF  THE  SOIL. 

In  the  vicinity  of  Tucson  there  are  several  sharply  distinguishable  phys- 
iographic areas  which  have  well-defined  characteristics  as  to  soils,  water 
and  temperature  relations,  exposure  and  plant  covering.  For  the  present 
purpose  these  areas  may  be  described  as  follows:  (i)  Tumamoc  Hill,  upon 
which  the  Desert  Laboratory  is  situated;  (2)  the  flood-plain  of  the  Santa 
Cruz  river;  (3)  the  bajada  or  mesa* ;  (4)  West  Wash.  In  addition  to  these 
areas  a  portion  of  the  high  bajada  which  lies  about  15  miles  east  of  Tucson 
and  the  upper  bajada  slopes  at  the  western  base  of  the  Rincon  mountains, 
an  altitude  about  1,300  feet  above  Tucson  and  20  miles  distant,  were 
observed.  These  areas  are  among  the  most  sharply  defined  of  all  of  those 
of  low  altitude  in  the  vicinity  of  Tucson. 

TUMAMOC  HILL. 

Tumamoc  Hill  is  a  low  mountain  rising  about  800  feet  above  the  sur- 
rounding bajada,  and  is  an  isolated  member  of  the  Tucson  range.  The 
northern  slope  is  fairly  gentle;  the  other  sides  are  more  or  less  precipitous, 
A  wash  or  arroyo,  which  heads  in  the  southeastern  part  of  the  mountain, 
runs  along  the  eastern  base,  and  another  wash,  called  here  West  Wash,  is 
along  the  western  side.  Bold  outcrops  of  volcanic  rock  occur  on  the  west, 
south,  and  east  sides  particularly,  and  irregular  masses  of  rock  form  descend- 
ing steps  along  the  northeastern  face.  In  crevices  of  the  rock  and  incrust- 
ing  thinly  the  surface  of  the  upper  rocks  which  are  not  exposed  is  a  hardpan, 
the  caliche,  which  is  practically  impervious  to  water. 

The  soil  of  Tumamoc  Hill  is  an  adobe  clay,  malpais,  which  is  derived 
from  the  lava  rock.  It  contains  small  particles  of  volcanic  rock  and  of 
caliche,  about  equaling  the  malpais  in  volume.  The  soil  as  thus  constituted 
varies  greatly  in  depth.  In  places  it  merely  covers  the  rocks,  while  at 
others  it  lies  in  pockets  and  may  be  50  cm.  or  more  deep. 

FLOOD-PLAIN  OF  THE  SANTA  CRUZ  RIVER. 

The  flood-plain  of  the  Santa  Cruz  river  at  Tucson  is  about  a  mile  in  width. 
Toward  the  eastern  side  of  the  plain  the  river  has  in  recent  years  cut  a 
channel  3  to  5  meters  deep ;  formerly  the  water  which  washed  down  from 
the  mountains  to  the  south,  from  the  Tucson  mountains,  or  from  the  bajada 
to  the  east  and  north,  spread  over  the  entire  floor  of  the  flood-plain.  This 
alteration  of  the  river  course  has  probably  affected  the  water  table  of  the 
plain  in  a  marked  degree,  causing  it  to  sink  to  a  level  lower  than  that  for- 
merly occupied. 

The  soils  of  the  plain  have  been  exposed  along  the  banks  of  the  river  so 
that  examination  of  them  to  the  depth  attained  by  the  river  is  an  easy 

*The  term  bajada  is  used  to  designate  the  gently  sloping  masses  of  detrital  material, 
which  depend  from  the  bases  of  mountain  masses,  and  form  such  a  prominent  feature 
of  deserts. 


^t^^m^i^^^f^^ "' 


A.  Upper  soil,  adobe  clay,  of  Santa  Cruz  flood-plain,  over  4  m.  in 
thickness  at  the  place  shown. 

B.  Adobe  clay,  the  darker  soil  overlying  the  caliche  hardpan  with 
"rotten"  caliche,  broken  masses  of  the  hardpan  between.  From  the 
bajada,  1  mile  east  of  the  Santa  Cruz  River. 


ENVIRONMENT   OF   THE    ROOTS.  1 3 

matter;  and  through  the  digging  of  two  wells,  one  on  the  pump  lot  and 
one  on  the  experimental  garden  of  the  Desert  Laboratory,  both  within 
70  meters  of  the  western  side  of  the  flood  plain,  a  soil  section  to  a  depth 
exceeding  12  meters  has  been  obtained. 

Along  the  course  of  the  river  the  soils  are  adobe  clay  and  apparently 
homogeneous  to  the  depth  examined.  At  the  western  side  to  the  same 
depth,  that  is,  about  5  meters,  the  soil  is  of  a  character  similar  to  that  near 
the  river,  but  below  5  meters  the  adobe  gives  place  to  stratified  sands  and 
gravels.  Caliche  was  not  found  in  the  soils  of  the  flood-plain  to  the  depth 
observed  and  probably  none  exists  there. 

WEST  WASH. 

West  Wash  lies  along  the  western  base  of  Tumamoc  Hill  and  separates 
it  from  the  benches  of  the  bajada  which  stretch  farther  west  to  the  main 
part  of  the  Tucson  mountains.  It  receives  the  drainage  from  the  western 
face  of  Tumamoc  and  from  a  small  portion  of  the  Tucson  range.  The 
wash,  for  the  present  purposes,  may  be  said  to  be  differentiated  into  a 
channel  from  5  to  10  meters  in  width  and  a  small  plain  which  separates 
the  channel  from  the  benches  to  the  west. 

The  soil  of  the  channel  is  a  coarse  sand  which  reaches  to  an  undetermined 
depth.  The  soil  of  the  plain  is  a  sandy  loam  to  a  depth  exceeding  2  meters. 
In  both  channel  and  plain  the  water  quickly  disappears  from  the  surface ; 
hence,  as  will  appear  repeatedly,  the  conditions  of  plant  life  here  are  mark- 
edly different  from  those  of  the  river  flood-plain,  as  well  as  from  those  of 
the  other  habitats  to  be  described. 

The  only  plant  seen  growing  in  the  channel  of  the  wash  was  a  specimen 
of  Curcubita  digitata,  with  a  fleshy  root.  Along  the  banks  of  the  channel, 
which  are  less  than  a  meter  below  the  level  of  the  adjacent  plain,  there  is  a 
fairly  heavy  growth  of  Acacia  constricta,  A.  greggii,  Covillea  tridentata, 
Ephedra  trifurca,  Parkinsonia  torreyana,  Prosopis  velutina,  and  Zizyphus 
parryi,  with  occasional  specimens  of  Echinocacius  wislizeni.  The  flood- 
plain  of  the  wash  contains  all  the  species  named  as  occurring  along  the 
channel,  but  the  growth  is  more  scattering  and  the  plants  may  be  some- 
what smaller. 

THE  BAJADA. 

The  bajada  is  the  drainage  slope  of  the  mountains  and  constitutes  the 
mesa,  or  Covillea  plain.  In  places  in  this  vicinity  it  extends  in  a  gentle 
gradient,  said  to  be  about  4  per  cent,  for  distances  of  10  miles  or  more. 
The  slope  of  the  bajada  from  the  base  of  a  mountain  range,  when  viewed 
in  profile  and  at  a  distance,  constitutes  one  of  the  most  striking  features 
of  desert  topography.  Where  the  slope  of  the  bajada  is  short  there  are 
practically  no  cross-drainage  channels,  but  where  it  comprises  a  wide  extent 
of  territory,  physiography,  soils,  and  other  physical  characteristics  peculiar 
to  it  are  developed  which  serve  to  greatly  increase  the  diversity  of  this 


14  ROOT   HABITS    OF   DESERT   PLANTS. 

formation.  No  physiographic  area  of  this  vicinity  has  so  great  diversity 
as  the  bajada. 

The  portions  of  the  bajada  brought  into  this  study  comprise  the  upper 
slope  at  the  north  base  of  Tumamoc  Hill,  the  drainage  slopes  west  of  West 
Wash,  the  high  mesa  about  a  mile  east  of  the  vSanta  Cruz  river,  a  certain 
locahty  about  15  miles  east  of  Tucson,  and  the  upper  reaches  of  the  bajada 
at  the  west  base  of  the  Rincon  mountains.  The  range  of  the  bajada  in 
altitude  at  the  stations  mentioned  runs  from  2,500  feet,  at  the  north  base 
of  Tumamoc,  to  about  3,500  feet  at  the  west  base  of  the  Rincons. 

The  soils  of  the  bajada  are  unhke  in  the  localities  mentioned,  but  a  study 
of  them,  aside  from  observations  in  the  field,  has  been  confined  to  that 
portion  which  lies  just  north  of  Tumamoc  Hill.  The  soils  of  the  different 
localities,  however,  have  certain  features  in  common,  some  of  which  are  as 
follows :  The  upper  soil  layer,  to  a  depth  of  30  cm.,  more  or  less,  is  of  adobe 
clay.  Underlying  the  adobe  is  caliche,  a  calcareous  hardpan,  which  extends 
to  an  indefinite  depth.  The  lower  portion  of  the  adobe,  perhaps  10  cm., 
is  composed  of  fragments  of  caliche  and  frequently  of  rock,  and  for  con- 
venience is  here  referred  to  as  "rotten"  cahche.  It  is  a  common  occur- 
rence that  the  caliche  hardpan  is  cracked,  or  is  wanting  in  small  areas,  so 
that  the  adobe  which  replaces  it  is  consequently  of  considerable  thickness. 
Plate  I,  taken  from  a  photograph  of  a  cut  in  the  bajada  a  mile  east  of  the 
river  flood-plain,  shows  variation  in  the  depth  of  the  soil  which  had  been 
brought  about  in  the  manner  indicated.  The  adobe  is  also  of  greater  depth 
where  it  has  accumulated  in  depressions  as  a  result  of  the  erosion  of  higher 
areas. 

Although  certain  general  conditions  arc  shared  by  all  of  the  bajada  soils 
examined,  even  a  superficial  examination  and  comparison  of  them  shows 
great  diflferences.  On  the  portions  of  the  bajada  where  the  distance  from 
the  mountains  is  relatively  great,  the  adobe  is  practically  homogeneous; 
but  near  the  base  of  the  mountains,  as  at  the  north  base  of  Tumamoc,  it 
may  be  shot  through  with  fine  rock  fragments  and  caliche  which  may  about 
equal  half  the  volume  of  the  whole.  On  the  slopes  west  of  West  Wash 
there  is  little  adobe,  and  what  is  comparable  to  the  rotten  caliche  of  the 
other  parts  of  this  formation  there  comes  very  close  to  the  surface,  thus 
causing  a  larger  percentage  of  caliche  and  rock  fragments  at  this  place. 

A  special  characterization  of  the  soil  conditions  in  the  habitats  of  each 
plant  examined  will  be  given  with  the  account  of  the  root-system  of  the 
plant. 


MOISTURE  IN  THE  SOIL. 
TUMAMOC  HILL. 

The  water  relations  of  the  plants  of  Tumamoc  Hill  are  in  part  condi- 
tioned b)'-  the  configuration  of  the  hill,  in  part  by  the  character  of  the  soil 
and  the  vegetal  covering,  and  in  part  by  its  relation  to  the  rest  of  the 
Tucson  mountains. 

Tumamoc  Hill  is  an  outlying  spur  of  the  Tucsons  and  is  lower  than 
many  peaks  of  this  range,  for  which  reason  the  rainfall  on  Tumamoc  is  prob- 
ably less  and  the  temperature  probably  higher  than  on  the  more  elevated 
parts  of  those  mountains.  Owing  to  its  isolation,  Tumamoc,  although  lower 
than  the  rest  of  the  range  to  which  it  belongs,  does  not  profit  from  the  run- 
oft'  from  these  mountains. 

Tumamoc  is  flat-topped  and  has  a  gently  sloping  northern  side,  where 
there  are  two  or  more  shoulders,  and  sharply  descending  eastern,  southern, 
and  western  faces.  The  soil  is  deepest  where  the  slope  is  least,  that  is, 
on  the  northern  shoulders.  Here,  also,  because  of  the  slight  gradient  and 
because  of  seepage  and  superficial  run-off  from  higher  portions  of  the  Hill, 
the  water  relations  are  most  favorable. 

Besides  these  factors  the  character  of  the  rains,  which  are  seasonal  and 
often  torrential,  and  the  want  of  a  heavy  plant-covering  operate  to  render 
much  of  the  water  which  falls  of  no  avail  to  the  plants  on  the  hill.  The 
rainfall  and  its  amount  and  character  for  a  period  of  15  years  at  the  city  of 
Tucson  are  given  month  by  month  in  the  following  table.* 

Table  i.- — Rainfall  at  Tucson  (in  Inches). 


Month. 

Rainfall. 

Month. 

Rainfall. 

January . .  . 

0.79 

July 

2.40 

February' . . 

0 .  90 

August 

2.60 

March 

0.27 

September. . .  . 

1. 16 

April 

0.14 

October 

0.64 

May 

0.14 

November.  .  .  . 

0.87 

June 

0 .  26 

December .... 

1 .00 

Thus  the  heaviest  fall  of  rain  occurs  in  midsummer  and  in  midwinter, 
with  little  or  no  rainfall  between.  The  character  of  the  rains  of  the  two 
seasons  is  unUke,  that  of  summer  is  frequently  torrential,  while  that  of 
winter  comes  with  less  force.  Much  of  the  rain  in  the  summer  season, 
therefore,  is  lost  as  superficial  run-off.  Consequently,  rain  falling  in  equal 
amounts  in  winter  and  in  summer  probably  produces  unequal  effects  in 
moistening  the  soil  and  in  providing  the  plants  with  water. 

The  penetration  of  the  soil  by  the  rain  is  not  a  fixed  amount,  but  depends 
on  a  variety  of  factors,  among  which  are  the  amount  and  duration  of  the 
rain.     The  depth  of  penetration  is  usually  from  2  to  5  times  the  registered 

*Coville  and  MacDougal:  The  Desert  Botanical  Laboratory,  Publication  No.  6, 
Carnegie  Institution  of  Washington,  1903,  p.  26. 


1 6  ROOT   HABITS    OF    DESERT   PLANTS. 

precipitation,  but  the  total  penetration  naturally  does  not  exceed  the 
thickness  of  the  adobe,  that  is,  50  cm.  more  or  less,  usually  less. 

The  studies  on  the  movements  of  the  moisture  of  the  soil,  on  which  this 
account  is  mainly  based,  comprise  observations  from  October  3,  1907,  to 
April  II,  1908,  and  include,  therefore,  only  the  dry  autumn  and  dry  early 
winter,  the  winter  rainy  season  and  the  earh^  spring.  Studies  on  the  condi- 
tions for  the  remainder  of  the  year  are  not  now  available.  The  special 
studies  were  made  on  soils  at  two  depths,  namely,  15  and  30  cm.  The  soil 
was  removed  at  frequent  intervals,  placed  in  stoppered  bottles,  and  dried 
in  an  oven  at  the  laboratory. 

The  following  summary  gives  the  water  movements  during  the  period 
under  consideration,  together  with  the  rainfall  recorded  on  Tumamoc  Hill 
at  the  time: 

From  October  until  the  latter  part  of  January  1.68  inches  of  rain  fell, 
and  the  curves  of  soil  moisture  at  this  time  were  fairly  flat;  that  for  the 
deeper  soils  fluctuated  less  than  that  for  the  shallow  soils.  Between 
January  13  and  February  11,  1.54  inches  of  rain  were  reported  at  the 
laboratory.  The  soils  on  the  first  of  February  were  the  driest  of  the  period 
under  observation.  At  the  15  cm.  depth  they  contained  15  per  cent  water ; 
at  the  depth  of  30  cm.,  14.5  per  cent  (dry  weight).  During  the  period 
between  February  i  and  February  1 1 ,  when  the  next  soil  observation  was 
made,  1.49  inches  of  rain  fell.  On  February  11  the  amount  of  water  in 
the  soil  at  the  15  cm.  depth  was  34.8  per  cent,  which  was  the  largest  amount 
at  that  depth  found  at  any  time  during  the  season.  At  this  time  there  was 
19. 1  percent  water  at  the  30  cm.  depth.  Between  February  1 1  and  March  23, 
0.87  inches  of  rain  fell,  in  small  amounts.  During  this  period  the  amount 
of  water  in  the  soil  at  the  higher  level  had  fallen  to  18.9  per  cent,  but  the 
water  content  at  the  depth  of  30  cm.  had  slightly  increased. 

Although  the  data  for  the  year  are  not  available  to  show  it,  it  is  fair  to 
assume  that,  as  the  dry  summer  approached,  the  soils  at  both  depths 
became  more  dry  and  reached  their  extreme  desiccation  in  July  when  the 
summer  rains  began.  The  per  cent  of  water  in  the  soils  at  the  time  is  not 
known,  but  observations  on  the  soils  at  the  laboratory  made  4  years  pre- 
viously in  July,  gave  the  following  results:  The  water  content  was  17.83 
per  cent  at  a  depth  of  30  to  40  cm. ;  at  a  depth  of  40  cm.  it  was  15.8  per 
cent  of  the  dry  weight  of  the  soil.  At  the  same  time,  at  the  15  cm.  depth 
the  moisture  content  was  only  9.1  per  cent.  The  significance  of  these  last 
figures  becomes  apparent  when  it  is  recalled  that  the  minimum  moisture 
content  of  this  soil  which  can  be  available  to  plants  not  possessed  of  water- 
storage  organs  is  10  per  cent  its  dry  weight. 

From  these  data,  admittedly  inadequate,  we  can  understand  some  of 
the  characteristics  of  the  environment  and  the  conditions  of  growth  of  the 
flora  of  Tumamoc  Hill.  The  soil  to  a  depth  of  15  cm.  probably  does  not 
retain  sufficient  moisture  for  absorption  by  plants  whose  roots  do  not 


MOISTURE    IN   THE    SOIL.  17 

reach  deeper  than  this,  for  a  period  much  exceeding  six  weeks  following 
storms.  This  defines  the  limit  of  life  of  most  annuals,  both  those  of  winter 
and  of  summer,  and  probably  also  the  season  of  absorption  of  perennials 
with  shallowly  placed  roots.  Plants  having  roots  which  reach  to  greater 
depths  than  15  cm.  can  obtain  some  moisture  at  all  seasons.  In  order  to 
survive,  seedlings  must  send  their  roots  below  15  cm.  within  six  weeks 
following  the  close  of  a  stormy  period. 

THE  BAJADA. 

The  water  relations  of  the  bajada  are  very  diverse,  because  of  differences 
in  soils,  in  topography,  and  in  its  relations  to  other  physiographic  areas. 
The  nearly  impervious  caliche  hardpan  which  underlies  the  bajada  ever}-- 
where  prevents  at  once  the  deep  penetration  of  the  rains  and  the  oppor- 
tunity of  tapping  subterranean  water.  Save  where  the  bajada  constitutes 
a  drainage  slope  from  a  higher  area,  and  well-defined  channels  are  not 
formed,  the  only  water  available  to  the  plants  growing  on  it  is  what  falls 
upon  it  directly.  The  water  table  of  the  bajada  varies  considerably  in  its 
position  with  relation  to  the  surface  of  the  ground.  That  in  the  vicinity 
of  the  University  of  Arizona,  about  a  mile  east  of  the  Santa  Cruz  river,  is 
approximately  25  meters  deep.  In  other  portions,  where  the  general  level 
of  the  bajada  is  higher,  the  perennial  water  is  considerably  lower  than  this 
figure. 

The  depth  of  the  penetration  of  the  rains  is  apparently  entirely  condi- 
tioned on  the  thickness  of  the  upper  soils  and  on  the  presence  of  cracks  or 
rifts  in  the  caliche  hardpan  which  are  filled  with  adobe.  In  favorable 
places  on  the  mesa,  where  there  has  been  a  considerable  accumulation  of 
adobe  soil,  enough  water  is  present  to  allow  the  growth  of  small  specimens 
of  such  perennials  as  are  most  abundant  on  the  flood-plain,  such  as  Prosopis 
velutina,  Acacia  greggii,  and  Ephedra  tr  if  urea. 

Where  the  bajada  nears  higher  areas,  as  Tumamoc  Hill,  the  most  favorable 
water  relations  are  to  be  found.  This  may  be  attributed  partly  to  the 
larger  rainfall  in  such  areas,  but  mainly  to  seepage  and  to  superficial  run-off 
from  the  higher  ground .  The  adobe  soil  from  the  bajada  north  of  Tumamoc 
Hill  has  an  admixture  of  fragments  of  caliche  and  of  rock  so  that  its  moist- 
ure-retaining capacity  is  about  20.1  per  cent  of  its  dry  weight.  For  this 
reason  it  is  air-dry,  to  a  depth  of  20  cm.,  most  of  the  year,  and  the  perennials 
which  are  to  be  found  on  it,  mainly  Covillea  tridentata  and  certain  cacti, 
must  get  their  water  during  dry  seasons  from  the  rotten  caliche  stratum 
and  from  the  more  deeply  placed  cracks  in  the  underlying  caliche  itself. 

During  the  period  in  which  observations  were  made  on  the  water  content 
of  the  soils  of  the  bajada,  October  3,  1907,  to  April  11,  1908,  it  was  learned 
that  there  was  much  less  water  in  the  soil  on  the  bajada  than  in  the  soils 
on  the  Hill.  Soil  samples  were  taken  at  two  depths,  10  cm.  and  20  cm. 
From  the  beginning  of  the  study  until  February  11,  the  water  in  the  soil 
at  either  depth  was  less  than  lo  per  cent  of  its  dry  weight.     On  February 


1 8  ROOT   HABITS   OF   DESERT   PLANTS. 

1 1  the  sample  at  a  depth  of  lo  cm.  had  17.3  per  cent  water,  and  that  from 
a  depth  of  20  cm.  had  18.6  per  cent  its  dry  weight.  Ten  days  later  the 
percentages  had  fallen  to  11.7  and  14.3,  respectively,  for  the  two  depths. 
Following  this  date  the  amount  of  water  diminished  rapidly  until,  four 
weeks  after  the  maximum  water  content,  it  was  no  more  than  immediately 
before.  The  readings  on  March  12  for  the  depths  of  10  and  20  cm.  w^ere 
7.9  and  9.5  per  cent  dry  weight.  This  was  considerably  under  the  amounts 
of  water  found  on  Tumamoc  at  the  time  and  shows  the  bajada  to  be  the 
more  arid  of  the  two  areas. 

The  period  during  which  the  adobe  clay  of  the  bajada  at  this  situation 
contained  sufficient  water  to  be  of  use  to  shallow- rooted  plants  was  thus 
not  more  than  three  weeks  of  the  year  in  question.  This  period  would 
probably  be  somewhat  extended  and  might  be  comparable  to  the  Tumamoc 
Hill  soils  in  portions  of  the  bajada  where  the  soil  contains  less  rock  and 
caliche  fragments  and  also  has  the  advantage  of  position  near  higher  areas. 
The  brevity  of  the  period  in  which  the  bajada  soil  contains  sufficient  water 
for  the  growth  of  annuals,  or  of  perennials  without  water-storage  organs, 
is  therefore  an  important  feature  of  the  xerophilous  conditions  of  this 
physiographic  area  and  indicates  that  it  is  the  most  intensely  arid  of  any 
examined. 

THE  WEST  WASH. 

Tumamoc  Hill,  as  above  noted,  is  situated  apart  from  the  main  range  of 
the  Tucson  mountains.  At  both  the  east  and  the  west  bases  of  the  Hill 
are  washes,  of  which  the  former  heads  in  the  hill,  while  the  latter  receives 
drainage  also  from  portions  of  the  main  range.  The  West  Wash,  therefore, 
drains  a  large  area  and  during  seasons  of  heaviest  storms  carries  a  large 
amount  of  water  which  may  overflow  its  banks  and  flood  the  bordering 
plain. 

The  soil  of  the  wash  is  coarse  sand  and  allows  water  to  percolate  through 
it  very  rapidly.  It  is  possible  that  there  is  a  water  table  underlying  the 
wash  and  its  flood-plain,  but  this  has  not  been  demonstrated.  The  sandy 
loam  of  the  plain  has  a  water  capacity  of  about  25  per  cent  its  dry  weight, 
or  considerably  higher  than  that  of  the  upper  reaches  of  the  bajada. 

The  soils  of  the  plain  only  have  been  studied;  the  studies  were  made 
from  October,  1907,  to  April,  1908.  The  studies  on  the  loams  of  the  plain 
showed  that  up  to  February  11  they  contained  less  than  10  per  cent  of 
water.  On  February  11  the  soil  at  a  depth  of  15  cm.  contained  14.9  per 
cent.  At  a  depth  of  30  cm.  the  soil  during  the  entire  period  did  not  con- 
tain over  8.2  per  cent  water.  On  March  2  the  soil  at  the  lesser  depth  had 
9.3  per  cent  and  that  at  the  deeper  location  6.2  per  cent  water,  April  11 
the  percentages  of  water  for  the  two  levels  were,  respectively,  5.3  and  4.6. 
The  dryness  of  the  upper  soil  is  due  to  its  sand-loam  nature,  which  permits 
the  rapid  sinking  of  water  and  its  rapid  evaporation.  In  this  place  the 
soil  is  over  2  meters  deep,  and  had  studies  been  carried  out  on  the  deep 


MOISTURE   IN   THE   SOIL.  1 9 

soil,  as,  for  example,  2  meters  beneath  the  surface,  a  much  higher  water 
content  would  probably  have  been  found. 

The  relation  of  the  roots  of  the  plants  of  the  area  will  be  spoken  of  later 
in  the  paper,  but  it  may  be  mentioned  here  that  of  non-fleshy  forms  only 
those  with  deeply  penetrating  roots  are  to  be  found,  and  that  only  here 
does  a  deeply  penetrating  root  develop  in  a  thoroughly  normal  and  typical 
manner. 

FLOOD-PLAIN  OF  THE  SANTA  CRUZ  RIVER. 

The  conditions  of  soil  moisture  of  the  river  flood-plain  are  relatively 
favorable  for  plants,  perhaps  the  most  so  of  any  area  under  discussion, 
which  is  partly  owing  to  the  character  of  the  soil  and  partly  to  the  com- 
paratively level  surface.  In  earlier  times  the  flood  waters  covered  the 
plain,  making  of  it  a  cienega ;  but  comparatively  recently  the  river  has  cut 
a  channel  and  the  only  flood  waters  that  go  over  the  plain,  at  times  not 
inconsiderable,  are  derived  from  the  adjoining  bajada.  The  river  channel 
carries  water  only  part  of  the  year,  and  yet  during  seasons  of  storms  it  may 
be  a  yellow  torrent  with  great  erosive  power.  The  water  table  of  the 
flood-plain  lies  from  about  5  meters,  near  the  channel  of  the  river,  to  about 
12  meters  at  the  western  edge. 

The  top  soil  of  the  plain  is  an  adobe  loam  with  a  water-retaining  capacity 
of  about  38.5  per  cent  its  dry  weight,  the  largest  retaining  capacity  of  any 
soils  of  the  different  habitats  so  far  examined. 

The  digging  of  a  well  on  the  western  side  of  the  flood  plain  afforded  an 
opportunity  of  determining  the  water  content  of  soil  to  a  depth  of  5.25 
meters,  although  the  upper  adobe  stratum  did  not  attain  a  greater  depth 
than  5.10  meters.  It  was  found  that  the  water  content  increased  with 
depth  to  the  limits  of  the  adobe.  Following  are  the  determinations:  At 
the  depths  of  0.20,  3.30,  4.00,  and  5.10  meters  the  water  contents  per  dry 
weight  of  soil  were  12.9,  19.0,  22.6,  and  23.1  per  cent  respectively.  At  the 
depth  of  5.25  meters,  where  sand  was  encountered,  the  water  content  was 
7.1  per  cent.  It  would  appear  from  these  determinations,  therefore,  that 
sufficient  water  is  present  in  the  adobe,  at  whatever  depth  examined,  to  be 
of  use  to  plants,  probably  throughout  the  year.  But,  as  will  appear  repeat- 
edly in  later  portions  of  the  study,  the  plants  at  the  present  time  do  not  as 
a  rule  actually  penetrate  to  the  deeper  levels  except  only  close  by  the  river ; 
for  the  most  part,  the  roots  are  confined  to  the  upper  two  meters  of  soil. 

Determinations  of  the  water  in  the  plain  soils  were  made  from  October 
3,  1907,  to  April  II,  1908,  and  revealed  the  fact  that  the  moisture  content 
of  the  soil  at  both  depths  taken  for  study,  15  and  30  cm.,  was  greater  most 
of  the  time  than  10  per  cent  its  dry  weight.  The  sample  taken  February 
II  showed  a  moisture  content  of  26.00  per  cent  at  the  depth  of  15  cm. 
From  this  date  until  April  1 1  the  water  content  gradually  fell  away  until 
it  became  10.9  and  11.6  per  cent  for  the  two  depths  respectively. 


TEMPERATURE  OF  THE  SOIL. 

The  observations  on  the  temperature  of  the  soil  have  been  confined  to 
two  series,  namely,  to  a  continuous  one  by  means  of  thermographs  and  to 
a  fairly  large  number  of  readings  of  thermometers .  The  thermometric  read- 
ings are  for  depths  of  i,  2,  and  12  inches.  The  thermograph  records  date 
from  the  summer  of  1905  and  the  spring  of  the  following  year,  and  give  the 
temperature  for  one  station,  close  by  the  laboratory  building,  on  the  north 
slope  of  Tumamoc  Hill,  and  for  two  depths,  15  and  30  cm. 


RECORD  OF  THERMOGRAPH:  15  CM.  DEPTH. 

An  almost  continuous  record  of  the  temperature  of  the  soil  at  the  15 
cm.  depth  is  at  hand  for  the  years  1905,  1906,  1907,  1908,  and  1909.  An 
examination  of  these  temperature  graphs  shows  interesting  features,  some 
of  which  will  be  noted. 

The  record  shows  an  undulating  line  of  which  the  curve-crests  correspond 
to  the  warmest  for  each  day,  and  the  depressions  the  coldest.  The  crests 
for  any  record  (each  one  is  for  7  days)  are  remarkably  uniform  in  height,  as 
also  the  depressions  are  uniform  in  depth.  The  difference  between  the 
crests  and  the  depressions  is  about  8°F.,  with  12°  as  the  greatest  variation, 
which  usually  occurs  in  March  and  July.  Owing  to  the  lagging  of  the  soil 
temperatures,  as  compared  wdth  those  of  the  air,  the  maximum  at  this 
depth  is  not  attained  until  about  6  p.  m.  and  the  minimum  about  midnight. 

The  uniformity  of  the  daily  temperature  range  is  broken  during  stormy 
periods,  particularly  in  the  winter  season.  In  summer,  the  undulating 
curve  of  daily  variation,  even  during  stormy  periods,  may  be  identified, 
although  its  amplitude  is  greatly  decreased.  The  extreme  yearly  range  of 
temperature  for  this  depth,  for  any  year  since  the  records  began,  is  69°  F.  ; 
and  the  extreme  range  for  the  entire  period,  1905  to  1909,  is  73°  F.  The 
following  table  gives  the  greatest  range  in  temperature  for  each  year. 

Table  2. — Maximum  and  Minimum  Soil  Temperatures  at  a  Depth  of  15  cm. 


Year. 

Maximum. 

Minimum. 

Date. 

Temp. 

Date. 

Temp. 

1 

1906 July  18 

1907 July    2 

1908 July    6 

1909 July  12 

°F. 
98 
105 

lOI 

99 

Jan.    3 
Jan.    7 
Dec.  22 
Dec.  25 

°F. 

34 
36 

42.5 
42 

If  we  follow  the  course  of  the  temperature  record  throughout  the  year, 
we  shall  see  that  the  period  of  greatest  heat  is  during  the  latter  part  of 
July,  or  immediately  preceding  the  rainy  season  of  midsummer.  With  the 
advent  of  the  rains,  the  temperatures  suddenly  decline,  the  decline  con- 


TEMPERATURE   OF   THE   SOIL.  21 

tinuing  until  midwinter.  As  great  a  fall  as  14°  F.  has  been  recorded  in  a 
day.  In  January  the  temperature  begins  to  rise,  and  the  rise  is  gradual 
until  the  last  of  March,  when  it  becomes  accelerated,  so  that  by  May  the 
soil  approaches  the  temperature  characteristic  of  early  summer  and  is  prob- 
ably as  great  as  that  of  the  germinating  period  of  midsummer.  The  course 
of  the  temperature  for  the  3^ear  is  fairly  well  illustrated  by  that  of  the 
minima  for  1908.  From  January  4  to  March  29  the  minimum  lay  between 
50°  and  60°  F. ;  between  April  5  and  April  26  it  was  between  60°  and  70°; 
in  May  the  minimum  was  between  70°  and  80°;  in  the  first  half  of  June 
it  W'as  between  80°  and  90°,  and  from  then  until  July  15  the  thermograph 
did  not  record  any  temperature  under  85°.  With  the  coming  of  the  sum- 
mer rains  the  minimum  began  to  fall  and  was  near  80°  until  October  4; 
it  fell  to  about  60°  until  November  26.  The  minimum  was  about  50°  until 
December  14,  and  between  40°  and  50°  the  remainder  of  the  year. 

RECORD  OF  THERMOGRAPH :  30  CM.  DEPTH. 

The  soil  temperatures  for  the  depth  of  30  cm.  were  recorded  under  the 
same  conditions,  except  only  the  greater  distance  from  the  surface,  as 
those  of  the  15  cm.  depth  just  described.  The  sensitive  bulbs  of  both 
instruments  were  only  a  meter  apart.  It  needs  but  a  glance  at  the  records 
of  the  two  instruments  to  reveal  their  most  striking  difference :  the  daily 
curve  of  the  more  deeply  placed  instrument  has  slighter  undulations.  In 
addition,  the  annual  variation  of  the  two  and  the  course  of  temperature 
throughout  the  year  are  also  markedly  dissimilar. 

The  usual  daily  range  in  temperature  of  the  soil  at  a  depth  of  30  cm.  is 
about  2°  F. ;  the  maximum  daily  range  for  1908,  which  may  be  accepted 
as  approximately  the  maximum  for  the  period  under  observation,  was  a 
drop  of  4°  in  July.  But  as  this  followed  as  a  result  of  the  rains  it  may  be 
taken  to  represent  the  unusual  rather  than  the  usual  temperature  variation 
for  the  depth.  The  maximum  temperature  for  the  entire  period,  1905- 
1909,  occurred  July  3,  1907,  when  the  thermograph  recorded  99°F. ;  the 
minimum  was  on  January  3,  1907,  at  which  time  the  temperature  was  44°. 
The  annual  extremes  in  temperature  are  presented  in  table  3. 


Table  3. — Maximum  and  Minimum  Soil  Temperatures  at  a  Depth  of  30  cm. 


Year. 

Maximum. 

Minimum. 

Date. 

Temp. 

Date. 

Temp. 

1905.... 
1906. . . . 

1907 

1908 

1909 

Aug.    7 
July  23 
July    3 
July    4-7 
July  12-13 

°F. 
97 
95 
99 

96 

Dec.  23 
Tan.    4 
Jan.    3 
Dec.  21 
Dec.    5 

°F. 
49 
46 
44 
50 
50 

ROOT   HABITS    OP   DESRRT   PLANTS. 


The  course  of  the  temperature  for  the  year  at  the  depth  of  30  cm.  is 
somewhat  different  from  that  at  the  lesser  depth.  This  is  graphically- 
shown  by  the  figure  below,  which  was  made  from  an  inspection  of  the 
records  for  the  year  1 908-1 909,  and  is  accurate  only  in  general  form,  not  in 
detail,  but  illustrates  the  facts.  As  fig.  i  indicates,  the  highest  temperature 
for  the  year  is  in  July,  or  August,  just  prior  to  the  rains  of  midsummer. 
With  the  coming  of  the  rains  the  temperature  falls  and  continues  to  decline 
until  the  middle  of  March,  when  it  begins  to  rise  and  quickly  reaches  the 
degree  characteristic  of  summer.  There  is  thus  a  grand  maximum,  which 
precedes  the  time  of  the  germination  of  the  summer  annuals  and  the  active 
growth  of  the  summer  perennials,  and  a  grand  minimum  which  follows  the 
most  active  period  of  vegetation  of  the  winter  season.  Broadly  speaking, 
therefore,when  the  activities  of  the  two  large  classes  of  plants  are  awakened, 
there  is  seen  to  be  a  difference  of  approximately  30°  F.  in  soil  temperature 
at  the  30  cm.  depth. 


70 


60 


50 


Auff. 

Sep. 

Oct. 

Nov. 

Dec. 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

"^ 

/ 

^ 

\ 

/ 

/ 

V 

■N.,^ 

/ 

/ 

y 

Fig.  1. — Temperature  of  the  soil,  at  a  depth  of  30  cm.,  for  the  year  1908-1909. 
Desert  Laboratorj-. 

SOIL  TEMPERATURES:  2.5  CM.  DEPTH. 

The  only  available  records  of  temperatures  of  the  soil  at  less  depths  than 
those  above  given  are  thermometer  readings  made  at  a  station  less  than 
50  meters  from  the  location  of  the  thermograph  records  given  above.  These 
readings  extend  from  March  22  to  May  10,  1907,  and  owing  to  the  rather 
brief  period  covered  by  the  thermometer  record  a  discussion  of  it  per  se  is 
precluded.  However,  it  will  be  of  interest  to  compare  the  thermograph 
record  and  that  of  the  thermometer  for  the  same  days,  as  shown  in  table  4. 

The  most  interesting  facts  revealed  by  the  records  are  the  differences  in 
temperatures  which  the  soil  shows  at  any  moment,  and  the  relatively  high 
temperatures  of  the  least  depth.  During  the  days  of  which  the  temper- 
ature record  is  available  the  greatest  difference  of  temperature  between 
the  depths  referred  to  is  shown  in  the  following  summary :  At  the  30  cm. 
depth  the  variation  was  3°  F. ;  at  the  15  cm.  it  was  11°;  and  2.5  cm.  it  was 
40°.     The  greatest  difference  in  maximum  temperatures  at  any  moment 


TEMPERATURE    OF   THE    SOIL. 


was  observed  on  April  15,  when  there  was  a  variation  of  23.5°  between  the 
upper  two  levels ;  unhappily  the  record  for  the  greatest  depth  for  the  time  is 
missing,  or  the  difference  would  doubtless  be  much  more.  From  these 
records  it  appears  that  the  roots  of  a  plant  which  reach  as  deep  as  30  cm. 
may  at  one  moment  in  springtime  experience  a  range  of  temperature  as 
great  as  22.50°  F.,  and  in  other  portions  of  the  year  probably  a  much  greater 
range. 

Table  4. — Soil  Temperatures  at  Different  Depths,  March-May,  1907. 


Date. 

2.5  cm. 

15  cm. 

30  cm. 

Thermometer.* 

Thermograph. 

Thermograph. 

Min. 

Max. 

Min. 

Max. 

Min. 

Max. 

March  22 

March  25 

March  29 

April  12 

^P"1^5 

May  I 

Mays 

May  10 

°F. 
63 
50 
50 
71 

65 
65 
64 

°F. 

83-5 

90 

80 
103 
103 . 5 
98 
97 
97-5 

°F. 
62.5 
64 

58.5 

75 

74 

76 
76 

°  F.      !      °  F. 
88.5         64 
75              66.5 
69-5          63.5 

85         1      .... 

So         1      

84.5          75 
84               76.5 

°F. 
67 

67.5 
64.5 

77 

77.5 

79 

' 

" 

^Maximum  and  minimum  thermometers  were  employed  in  the  study. 

SUMMARY  OF  ENVIRONMENTAL  CONDITIONS. 

The  habitats,  which  have  been  studied  in  connection  with  the  observa- 
tions on  the  roots  of  plants  inhabiting  them,  comprise  mesa,  or  bajada, 
and  include  locations  on  the  flood-plain  of  the  vSanta  Cruz  river,  the  West 
Wash  (near  Tumamoc  Hill),  and  on  Tumamoc  Hill.  The  studies  on  the 
soils,  the  moisture  in  them  and  their  temperatures,  adopted  from  various 
reports,  have  been  limited  to  Tumamoc  Hill,  or  the  bajada  and  flood-plain 
near  by. 

The  surface  soils  are  an  adobe  clay  with  a  varying  admixture  of  rock 
fragments,  either  of  volcanic  origin  or  of  caliche,  and  the  leading  apparent 
differences  in  the  soil  are  dependent  on  the  amount  of  this  coarser  material 
present.  The  upper  soil  is  usuallyfrom  15  to  30  cm.  in  thickness.  Beneath, 
to  a  considerable  but  varying  depth,  lies  a  hard  pan  caliche.  Between 
the  upper  stratum  and  the  hardpan  is  an  intermediate  zone  in  which  there 
are  large  fragments  of  caliche,  or  other  rock,  mingled  with  adobe.  The 
intermediate  stratum  is  usually  not  more  than  20  cm.  in  thickness.  The 
total  soil  generally  available  for  the  roots  is  thus  50  cm.,  and  frequently 
(usually  on  the  bajada)  it  is  much  less  than  this. 

The  depth  of  perennial  water  varies  with  the  habitats,  although  on 
Tumamoc  Hill  no  water  table  exists.  On  the  bajada  it  lies  25  meters  or 
more  beneath  the  surface,  and  on  the  flood-plain  of  the  river  the  water  table 
is  5  meters  deep,  or  more. 


sfe  t:%  :^ 


24  ROOT   HABITS    OF    DESERT   PLANTS. 

The  rainfall  on  each  of  the  areas,  in  the  vicinity  of  Tucson,  is  approxi- 
mately the  same,  but  the  differences  in  the  mutual  relation  of  the  areas, 
the  character  of  the  soil,  and  its  depth,  as  well  as  the  surface  configuration, 
operate  to  make  the  water  relations  of  the  habitats  very  unlike. 

The  flood-plains  of  the  vSanta  Cruz  river  and  of  West  Wash,  in  addition 
to  the  rains  which  fall  on  them  directly,  receive  the  run-off  and  seepage 
waters  from  the  bajada,  and  the  bajada,  in  turn,  those  from  Tumamoc  Hill 
or  other  higher  elevations.  In  former  years  at  flood-time  the  Santa  Cruz 
river  overran  its  banks  and  covered  the  bottom  lands,  but  now  the  river 
has  cut  a  deep  channel  and  flooding  from  this  cause  never  takes  place. 
The  change  is  probably  associated  with  the  cutting  of  the  mesquite  forest, 
which  was  once  a  rather  heavy  one,  and  the  conversion  of  the  bottoms 
into  ranches  or  waste  lands,  with  second-growth  mesquite. 

The  water-retaining  capacity  of  the  soils  of  the  river  flood-plain  is  the 
highest,  below  which  should  be  placed  the  adobe  of  the  bajada,  at  the  north 
base  of  the  Hill,  with  the  loamy  sand  of  the  flood-plain  of  West  Wash  as  the 
least. 

So  far  as  is  known  from  studies  on  the  water  content  of  the  superficial 
soils,  the  amount  of  water  present  during  the  dry  autumn  and  winter  on 
the  bajada  north  of  Tumamoc  and  on  the  flood-plain  by  West  Wash  is 
insufficient  to  be  of  direct  benefit  to  the  plants  whose  roots  do  not  penetrate 
deep,  while  at  the  same  time  the  soils  of  Tumamoc  Hill  and  of  the  river 
flood-plain  have  moisture  present  in  usable  quantity. 

The  period  of  optimum  moisture  content  of  the  upper  soil  includes  the 
rainy  period  and  a  relatively  short  time  after  the  beginning  of  the  dry 
season.  The  upper  levels  dry  out  soonest,  and  on  the  bajada  were  air-dry 
three  weeks  after  the  close  of  the  rains,  while  the  soils  on  the  river  flood - 
plain  and  Tumamoc  Hill  remained  moist  for  a  period  exceeding  six  weeks. 
This  places  the  limit  to  the  growing  period  of  most  annuals  and  the  most 
active  vegetative  period  of  all  perennials  without  water  storage  capacity, 
or  deeply  penetrating  roots. 

The  temperature  of  the  soil  has  been  observed  on  Tumamoc  Hill  only, 
and  chiefly  at  15  cm.  and  30  cm.  The  highest  temperatures  immediately 
precede  the  summer  rains.  With  the  advent  of  the  rains  the  temperature 
falls  quickly  and  continues  to  decHne  until  January-February,  at  a  depth 
of  15  cm.,  or  March- April  at  the  greater  depth — 30  cm.  The  time  of  the 
germination  of  the  summer  annuals  and  of  greatest  vegetative  activity  of 
the  perennials,  therefore,  is  below  the  maximum  heat,  as  that  of  the  winter- 
spring  plants  is  above  the  minimum. 

The  daily  fluctuations  of  temperature  for  the  two  depths  is  unlike ;  that 
at  the  greater  depth  is  about  2°  F.,  while  that  at  the  lesser  depth  is  about 
8°  F.  The  yearly  range  at  the  15  cm.  depth  is  about  34°,  and  that  at  30 
is  about  44°  F.,  with  the  maxima  at  99°  and  105°  for  the  two  depths 
respectively. 


a.   Amaranthus  palmeri,  Tumamoc  Hill,  August  10,  1909. 
b    Anoda  thurberi,  Tumamoc  Hill,  August  10,  lOOO. 

c.  Aster  tanacetifolius,  Tumamoc  Hill,  August  26,  1909. 

d.  Aster  tanacetifolius  showing,  at  left,  root-formation  at  base  of  lateral  of 
first  order.  The  figure  at  right  has  segment  of  tap-root  bearing  lateral  of  first 
order  with  its  branches. 


ROOT  HABITS  OF  DESERT  PLANTS. 
In  presenting  descriptions  of  the  root-systems  of  the  desert  plants,  it 
seems  best  to  be  guided  by  the  biological  grouping — that  is,  to  bring  plants 
together  which  are  naturally  segregated  either  in  time  or  in  space.  Accord- 
ingly perennials  which  arc  typical  of  the  leading  habitats  treated  are 
grouped  under  the  respective  habitats,  but  since  the  annuals  of  winter  and 
of  summer  occupy  the  same  areas  and  are  separated  only  by  time  they  are 
divided  into  winter  and  summer  forms.  By  summer  annuals  is  meant 
those  which  appear  in  midsummer,  and  by  winter  annuals  those  which 
come  after  the  w^inter  rains,  although  some  of  them  might  more  truly  be 
called  spring  annuals  than  those  of  winter. 

SUMMER  ANNUALS. 

In  early  July  the  different  habitats  show  only  perennials,  and  the  bare 
ground  between  them  shows  the  dried  remains  of  the  annual  vegetation  of 
the  preceding  rainy  season.  With  the  coming  of  the  rains  of  late  July  or 
August,  however,  the  appearance  of  the  land  is  suddenly  changed.  Seeds 
which  have  lain  dormant  for  nearly  ii  months  promptly  germinate,  and 
hill  and  plain  are  speedily  clothed  with  a  growth  of  evanescent  forms  which 
are  in  great  variety  and,  for  desert  forms,  of  surprising  density. 

The  study  of  the  summer  annuals  was  carried  on  in  the  following  manner. 
The  roots  of  typical  forms,  mostly  mature,  were  carefully  washed  out  by  a 
small  jet  of  water,  and  the  entire  plant,  or  as  much  as  was  recovered  by  this 
means,  was  preserved  in  weak  formaline  for  subsequent  study.  By  this 
method  the  entire  root-systems  were  rarely  to  be  had,  but  enough  was 
always  saved  to  give  the  general  character.  More  than  one  plant  as  a  rule 
was  preserved  of  each  species  so  that  a  control  on  the  observations  might  be 
had.  A  very  serious  drawback  to  this  method  of  study  lay  in  the  impos- 
sibility of  surely  determining  the  extreme  depth  to  which  the  main  root 
(or  the  laterals)  penetrated  the  soil,  although  in  many  cases  this  feature 
was  learned  with  close  accuracy.  The  following  summer  annuals  were 
examined : 

Amaranthus  palmeri  S.  Wats.  Dysoda  papposa  Lag. 

Anoda  thurberi  Gray.  Euphorbia  glyptosperma  Engelm. 

Aster  tanacetifolius  HBK.  Kallstroemia  grandiflora  Torr. 

Boerhaavia  sp.  Pectis  prostrata  Car. 

Cladothrix  lanuginosa  Nutt.  Solanum  ela;agnifolium  Cav. 

Datura  sp.  Trianthema  portulacastrum  L. 
Ditaxis  humilis  (Engelm.  and  Gray)  Pax.         Vicia   sp. 
(perennial). 

Amaranthus  Palmeri. 

The  species  of  Amaranthus  studied  was  growing  near  the  Laboratory 
building  and  was  removed  from  the  soil  on  August  lo,  at  which  time  the 
shoot  was  21  cm.  long.     The  plant  was  not  in  flower. 

The  root-system  consists  of  a  tap  root  and  several  large  laterals  which 
bear  filamentous  branches.     The  roots  do  not  intergrade  in  size,  certainly 

25 


26  ROOT   HABITS   OF    DESERT   PLANTS. 

not  in  diameter.  Only  lo  cm.  of  the  main  root  was  recovered,  but  it  may 
have  been  much  longer  than  that,  since,  where  it  was  broken,  it  was  0.5  mm. 
in  diameter.  The  root  was  4  mm.  in  diameter  at  the  crown.  From  the 
main  root  5  leading  laterals  take  their  origin.  These  are  coarse  and  over  25 
cm.  long.  The  laterals  of  the  first  order  bear  numerous  long  delicate  roots, 
which  in  turn  are  branched.  The  ultimate  roots  are  long  and  filamentous, 
but  rudiments,  mere  root  tips,  are  also  very  numerous. 

The  root-system  of  Amaranthus  thus  is  characterized  by  two  classes  of 
roots,  coarse  and  fine,  without  intergradation,  and  b)''  the  presence  of  rudi- 
ments (plate  2). 

Anoda  Thurberi. 

The  specimen  of  Anoda  which  was  studied  was  procured  August  10;  it 
was  an  unbranched  shoot  22  cm.  in  height  and  bore  several  large  leaves. 
The  root-system  is  characterized  by  a  prominent  main  root  and  several  deli- 
cate secondary  ones.  The  tap  root  was  over  18  cm.  long  and  was  forked. 
From  the  tap  root  arise  numerous  laterals,  mostly  within  4  cm.  of  the 
surface  of  the  ground,  of  which  the  largest  is  over  6  cm.  in  length.  The 
laterals  of  the  second  order  bear  filamentous  branches.  No  rudiments  are 
present.  As  a  rule  the  roots  arise  singly,  although  a  few  of  the  ultimate 
ones  are  in  groups  (plate  2). 

Aster  Tanacetifolius. 

On  August  26  the  roots  of  Aster  were  removed  from  the  soil.  The  plant 
has  a  dense  habit  of  growth  and  bears  many  leaves.  The  shoot  was  25  cm. 
high.  The  tap  root  is  especially  well  developed.  It  is  4  mm.  in  diameter 
at  the  crown,  and  over  15  cm.  was  recovered.  The  laterals  form  a  dense 
tuft  3  to  6  cm.  beneath  the  surface  of  the  ground  and  are  uniformly  slender. 
There  are  no  rudiments. 

A  peculiarity,  not  observed  in  other  species,  was  the  place  of  origin  of  the 
laterals  of  the  second  order.  All  of  the  laterals  of  the  first  order,  which 
W'cre  in  the  dense  tuft  referred  to,  were  short — that  is,  less  than  3  cm.  long — 
and  the  tips  were  dead,  but  they  all  bore  long  and  delicate  branches,  as 
shown  in  plate  2 .  The  cause  of  this  was  not  definitely  ascertained,  although 
laterals  with  branches  near  the  somewhat  enlarged  base  were  seen  with 
the  tips  yet  living,  so  that  the  condition  noted  may  be  the  usual  occurrence 
in  the  species.     The  ultimate  roots  of  Aster  are  of  the  fourth  order. 

Below  the  tuft  of  absorbing  roots  just  described  are  scattered  slender 
laterals  arising  from  the  main  root.  The  root  habit,  with  the  relation  of 
the  laterals  to  the  main  root,  is  shown  in  plate  2. 

BoErhaavia  sp. 

The  specimens  of  Boerliaavia  examined  were  from  Tumamoc  Hill  near  the 
Laboratory  building  and  were  washed  out  August  10.  The  shoot  of  the 
plant  to  be  described,  which  was  in  bud,  was  12  cm.  high  and  bore  numerous 
large  leaves. 


ROOT   HABITS   OF   DESERT  PLANTS.  27 

The  main  characters  of  the  root-system  are  the  relatively  large  size  of  the 
laterals  of  the  first  order  and  the  paucity  of  filamentous  roots.  The  ulti- 
mate roots  are  of  two  sorts,  a  longer  and  a  shorter  kind.  The  latter  are 
rudiments  and  are  borne  in  the  axils  of  the  former;  they  are  shown,  but 
imperfectly  so,  in  plate  3.  The  plant  thus  shows  a  tendency  to  produce 
roots  in  groups. 

Cladothrix  Lanuginosa. 

The  specimens  of  Cladothrix  studied  were  removed  from  the  soil  on  August 
19,  and  were  mature  plants  with  large  leaves  and  shoots  about  19  cm.  in 
length.  The  main  root  was  over  2  mm.  in  diameter  at  the  crown  and  was 
6  cm. in  length  with  the  tip  atrophied,  but  it  was  continued  1 7  cm.  farther  by 
two  successive  laterals,  so  that  the  depth  attained  was  over  23  cm.  The 
laterals  are  numerous  and  rather  fine  and  for  the  most  part  arise  within  4 
cm.  of  the  surface  of  the  ground.  They  bear  laterals  of  the  second  order, 
which,  in  turn,  are  branched.  The  ultimate  roots  are  represented  in  very 
numerous  cases  by  rudiments. 

As  a  whole,  the  root-system  of  Cladothrix  is  characterized  by  the  slender- 
ness  of  its  roots,  a  large  number  of  which  are  filamentous.  Root  hairs  are 
present  in  great  abundance.  The  absorption  faculty,  therefore,  is  a  feature 
strongly  developed. 

Datura  sp. 

The  specimen  of  Datura  studied  was  collected  on  August  10  and  was 
immature.  The  shoot  was  15  cm.  in  length  and  bore  several  large  leaves. 
The  root-system  is  characterized  by  the  very  large  number  of  fine  roots, 
which  are  usually  long.  At  the  crown  the  main  root  is  4.5  mm.  in  diameter, 
but  tapers  rapidly  and  is  under  10  cm.  in  length.  At  a  point  8  cm.  beneath 
the  surface  of  the  earth  it  was  less  than  0.5  mm.  in  diameter.  The  laterals 
are  all  long  and  of  two  diameters,  a  few  are  noticeably  heavy,  and  man}^ 
are  very  slender.  All  of  the  laterals  of  the  first  order  bear  filamentous 
branches  which  are  long  and  branch  but  little.  The  roots  of  the  third  order 
are  especially  fine.  Plate  3  shows  only  imperfectly  the  density  of  the 
absorption  system  of  Datura. 

Ditaxis  Humilis. 

The  species  of  Ditaxis  examined  is  a  perennial  herb,  which  was  in  llower 
when  collected,  August  10.  The  shoot  is  leafy  and  over  12  cm.  long.  The 
roots  reach  deeper  than  those  of  the  other  herbaceous  forms  studied  and 
the  absorption  system  is  poorly  developed.  The  main  root  is  relatively 
slender;  it  is  over  23  cm.  in  length  and  is  only  2.5  mm.  in  diameter  at  the 
crown.  Four  large  and  numerous  small  laterals  of  the  first  order  arise 
within  8  cm.  of  the  surface  of  the  ground.  Very  few  laterals  are  given  off 
from  the  main  root  below  this.  The  laterals  may  extend  1 1  cm.  and  over, 
and  mostly  are  in  groups  of  three  or  four.  The  laterals  of  the  second  order 
are  filamentous  and  rarelv  branch.     There  are  no  rudimentarv  roots. 


28  root  habits  of  desert  plants. 

Dysoda  Papposa. 
Specimens  of  Dysoda  in  flower  were  procured  August  26.  The  shoots 
were  12  to  15  cm.  in  length,  much  branched,  and  well  covered  with  leaves. 
The  tap  root  dominates  the  root-system.  This  evidently  reaches  deep, 
although  unfortunately  but  10  cm.  of  the  root  was  uncovered.  The  laterals 
are  filamentous  and  are  given  off  from  the  main  root  somewhat  farther 
from  the  surface  of  the  ground  than  is  usually  the  case  in  summer  annuals. 
The  laterals  of  the  first  order  bear  branches  and  these  also  are  branched, 
but  the  entire  extent  of  the  filamentous  roots  is  relativel)'  slight  (plate  4). 

Euphorbia  Glyptosperma. 
Euphorbia  glyptospcnna  is  most  abundant  on  Tumamoc  Hill,  where  the 
specimens  examined  were  collected  August  26,  but,  like  most  of  the  other 
annuals  it  also  occurs  in  the  other  habitats  in  the  vicinity  of  the  Laboratory. 
The  specimens  studied  were  about  7  cm.  high,  much  branched,  with  a  large 
leaf  surface,  and  in  flower.  The  roots  grade  in  diameter  from  the  main  root, 
which  is  somewhat  over  i  mm.  in  diameter  at  the  crown,  to  the  ultimate 
filamentous  ones,  although  as  a  whole  the  roots  are  relatively  coarse.  The 
laterals  of  the  first  order  arise  for  the  most  part  within  3  cm.  of  the  surface 
of  the  ground.      They  occur  singly;  no  rudimentary  roots  were  observed. 

Kallstrcemia  Grandielora. 

Kallstramia  is  one  of  the  less  abundant  but  most  striking  of  the  summer 
annuals.  It  is  of  a  semiprostrate  habit,  which  varies  with  its  stage  of 
growth  or  with  the  conditions  under  which  it  has  developed.  If  the  summer 
rains  are  slight,  so  that  the  period  of  growth  of  the  plant  is  short,  its  habit 
is  nearlv  upright,  but  should  the  stormy  season  be  prolonged,  or  should 
rains  occur  after  those  of  early  summer  have  passed,  and  before  the  plant 
dies,  growth  is  renewed  if  the  latter  is  the  case,  or  in  the  former  case  growth 
is  so  active  that  the  plant  becomes  vine-like  and  prostrate.  Under  such 
favorable  circumstances  it  may  cover  as  much  as  16  square  meters  of 
surface,  and  perhaps  even  more  than  this.  The  plants  whose  roots  were 
removed  either  were  young  or  had  attained  the  first  stage  of  growth  de- 
scribed above,  that  is,  they  were  upright  and  mature. 

The  plant  to  be  described  was  collected  on  Tumamoc  Hill  on  August  10. 
The  shoot  consisted  of  two  branches  and  several  compound  leaves  and  more 
numerous  flower  buds.     It  was  27  cm.  long. 

The  root-system  consists  of  a  main  root,  with  a  relatively  stout  crown, 
and  one  long  and  one  short  branch,  together  with  slender  laterals,  which 
are  mainly  close  to  the  surface  of  the  ground,  and  their  proper  branches. 
The  longest  lateral  is  over  21  cm.  in  length  and  bears  two  branches  over  10 
cm.  each.  Most  of  the  laterals  of  the  first  order  and  all  of  the  higher  orders 
are  filamentous.     The  main  root  penetrated  the  ground  more  than  22  cm. 

The  leading  features  of  the  root-system,  which  is  a  rather  extensive  one, 
are  shown  in  plate  4, 


Boerhaavia  sp.     b.  Cladothrix  lanuginosa,     c.  Datura  sp.     d.   Ditaxis  humilis. 


a.  Dysoda  papposa,  flood-plain  of  Santa  Cruz.  b.  Euphorbia  glyptosperma, 
Tumamoc  Hill.  c.  Kallstroemia  grandiflora,  Tumamoc  Hill.  d.  Pectis 
prostrata,  Tumamoc  Hill.     e.  Solanum  elseagnifolium,  Tumamoc  Hill. 


root  habits  of  desert  plants.  29 

Pectis  Prostrata. 
The  specimens  of  Pectis  studied  were  growing  on  Tumamoc  Hill  near  J:he 
Laboratory  building.  They  were  from  7  to  12  cm.  in  length  and  in  flower. 
The  general  habit  of  the  plant  is  shown  in  plate  4.  The  main  root  is  very 
delicate  and  bears  several  long  laterals  within  3  cm.  of  the  surface  of  the 
ground.  The  longest  of  these  were  over  1 3  cm.  and  bore  numerous  branches, 
which  also  were  branched.  The  root-system  as  a  whole  may  be  character- 
ized as  being  little  branched,  the  laterals  of  all  orders  are  relatively  few, 
and  composed  of  delicate  roots  only. 

SOI.ANUM  El^AGNIFOLIUM. 

Immature  plants  of  Solanum  were  collected  August  22,  of  which  one  had 
a  shoot  6  cm.  high  and  bore  several  large  leaves.  A  tap  root  dominated  the 
root-system;  it  was  traced  over  15  cm.  Laterals  were  given  off  at  occasional 
intervals  along  as  much  of  the  main  root  as  was  examined,  but  they  were 
mainly  confined  to  the  upper  4  cm.  The  laterals  of  the  first  order  are  fine 
and  bear  filamentous  branches.  There  are  also  roots  of  the  third  order 
present.  The  roots  arise  singly;  there  are  no  rudiments.  The  leading  char- 
acteristics of  the  root-system  are  the  rather  stout  tap  root  and  the  delicate 
laterals,  with  no  roots  of  intermediate  diameter  between  (plate  4). 

Trianthema  Portulacastrum. 
The  specimens  of  Trianthema  which  were  studied  wTre  collected  on  the 
western  edge  of  the  Santa  Cruz  flood-plain,  August  26.  The  shoot  of  the 
plant  to  be  described  was  about  1 7  cm.  high  and  bore  9  branches  and  num- 
erous large  leaves,  and  therefore  possessed  a  large  transpiration  surface. 
The  root-system  is  characterized  by  a  preponderance  of  coarse  roots. 
The  tap  root  is  3  mm.  in  diameter  at  the  crown,  is  over  15  cm.  long,  and 
bears  (within  4  to  5  cm.  of  the  surface  of  the  ground)  5  prominent  laterals, 
the  longest  of  which  was  more  than  17  cm.  At  intervals  of  about  i  cm.  the 
laterals  of  the  first  order  bore  short  filamentous  roots  and  rudiments  of 
roots.  The  number  of  rudiments  is  large,  a  feature  which  distinguishes 
this  plant  from  nearly  every  other  one  examined.  The  rudiments  appear 
as  faint  dots  along  the  laterals  in  plate  5. 

Vicia  sp. 
The  specimen  of  Vicia  whose  roots  were  examined  was  growing  near  the 
Laboratory  and  was  collected  August  ip.  The  shoot  of  the  plant  was 
mature  and  bore  fruit  and  flowers.  The  specimen  studied  bore  7  branches 
and  numerous  linear  leaves.  The  root-system  is  characterized  by  a  prom- 
inent tap  root,  which  was  2  mm.  in  diameter  at  the  crown  and  over  20  cm. 
long,  and  large  laterals  arising  rather  far  beneath  the  surface  of  the  ground. 
A  portion  of  the  laterals  of  the  first  order  are  in  groups  of  2  or  3,  although 
they  mainly  arise  singly.  The  laterals  branch  but  little  and  leave  the 
impression  that  the  absorption  system  of  the  plant  is  a  rather  meager  one. 
A  few  tubercles  are  borne  on  the  larger  laterals. 


30  ROOT   HABITS    OF   DESERT   PLANTS. 

WINTER  ANNUALS. 

Between  the  months  of  November  and  March,  and  occasionally  one  month 
before  and  one  month  after  that  period,  if  the  moisture  conditions  are 
favorable,  a  large  number  of  annuals  make  their  appearance  in  the  different 
habitats  considered  in  this  study.  These  are  for  the  most  part  of  different 
genera  and  species  from  those  which  occur  in  summer,  and  like  the  latter 
have  to  a  large  degree  a  generalized  type  of  distribution.  Often  the  same 
species  of  Avinter  annuals  are  to  be  found  wherever  the  soil  and  the  water 
conditions  are  suitable  irrespective  of  the  habitat,  whether  Tumamoc  Hill 
or  the  lower-lying  bajada,  but  they  may  be  present  in  greatest  profusion  on 
the  lower  slopes  of  the  Hill.     The  following  winter  annuals  were  examined : 

Amsinckia  spectabilis  Fischer  and  Meyer.         Hordeum  murinum  L. 

Astragalus  nuttallianus  DC.  Malva  borealis  Wallm. 

Bowlesia  lobata  R.  and  P.  Malacothrix  clevelandi  Gray. 

Brodia'a  capitata  Benth.  Medicago  denticulata  Willd. 

Daucus  pusillus  Michx.  Mentzelia  albicaulis  Dougl. 

Eritrichium  pterocaryum  Torr.  Microseris  linearifolia  Gray. 

Erodium  cicutarium  (L.)  L'Her.  Monolepis  chenopodioides  Moq. 

Festuca  octoflora  Walt.  Orthocarpus  purpurascens  Benth. 
Gilia  bigelowii  Gray.  var.  palmeri  Gray. 

Harpagonella  palmeri  Gray.  Parietaria  debilis  Forst. 

Amsinckia  Spectabilis. 

.4  msinckia  spectabilis  is  one  of  the  most  abundant  of  the  winter  annuals 
and,  as  far  as  its  local  distribution  is  concerned,  one  of  the  most  generally 
distributed.  It  also  is  one  of  the  most  drought-resistant,  a  condition 
which  is  illustrated  by  the  following  note  on  its  behavior.  In  the  spring  of 
1907  one  of  the  staff  of  the  Desert  Laboratory  was  studying  the  invasion  of 
plants  into  a  denuded  area.  On  March  19  the  storms  of  winter  were  over 
and  the  arid  conditions  w^ere  rapidly  becoming  marked ;  on  that  date  the 
temperature  out-of-doors  in  the  shade  was  94°  F.  with  a  relative  humidity 
of  20  per  cent.  The  winter  annuals  were  for  the  most  part  already  dead. 
On  the  periphery  of  the  denuded  plot  mentioned  there  had  been  a  heavy 
growth  of  Sisymbrium  reflexum,  Bowlesia  lobata,  Amsinckia  spectabilis,  and 
Phacelia  tanacetifolia.  Of  these  annuals  the  first  two  were  nearly  all  dead 
and  were  shedding  their  seed,  while  the  last  two  were  apparently  unaffected 
by  the  drought,  though  all  of  these  plants  were  apparently  under  similar 
conditions. 

Amsinckia  has  a  deeply  penetrating  main  root  and  numerous  laterals, 
all  of  which,  especially  in  the  young  plants,  are  slender  and  branch  but 
little.  The  root-system  is  extensive  rather  than  intensive.*  As  the  plant 
becomes  older  the  tap  root  becomes  heavier,  and  the  difference  in  diameter 
between  it  and  the  laterals  is  somewhat  accentuated. 

As  in  the  summer  annuals,  the  length  of  the  main  root  as  given  in  this 
paper  is  usually  somewhat  less  than  the  actual  length,  owing  to  the  diffi- 

*Where  roots  are  thus  characterized  it  is  not  intended  to  classify  them  after  Biisgen, 
unless  so  stated,  but  the  words  are  merely  used  in  a  descriptive  sense. 


ROOT  HABITS   OF   DESERT   PLANTS.  3^ 

culty  in  removing  the  root-system  and  keeping  it  intact.  However,  great 
care  was  always  taken  to  obtain  as  much  of  the  roots  as  possible.  A  shoot 
of  a  mature  plant  of  Amsinckia  13.5  cm.  in  length  had  a  tap  root  over  8  cm. 
long;  a  plant  with  a  shoot  18  cm.  long  had  a  tap  root  more  than  12.5  cm. 
in  length.  Thus,  although  the  greatest  penetration  was  not  learned,  it  w^as 
sufficient  to  enable  the  plant  to  reach  and  to  tap  a  relatively  good  water 
supply  at  a  time  when  the  upper  layers  of  the  soil  were  too  dry  to  provide 
a  sufficient  amount  of  water  for  plants  whose  roots  were  entirely  restricted 
to  the  surface  soil  layers  (plate  5). 

Astragalus  Nuttallianus. 

In  this  vicinity  Astragalus  grows  under  a  large  variety  of  conditions  as 
regards  exposure,  kinds  of  soil,  and  relations  to  other  plants.  It  is  practi- 
cally cosmopolitan  in  its  local  distribution.  A  mature  plant  with  a  shoot 
6  cm.  long,  together  with  young  specimens,  was  preserved  for  examination. 
The  leading  character  of  its  root-system  was  the  well-developed  tap  root, 
13  cm.  in  length.  The  laterals  were  few  and  coarse,  and  little  branched. 
Tubercles  were  present  in  considerable  abundance. 

BowLESiA  Lobata. 

Boivlesia  is  to  be  found  on  Tumamoc  Hill  and  on  the  lower  detrital  slopes, 
and  usually  occurs  under  the  protection  of  a  shrub  or  tree  or  in  other  situ- 
ations where  the  water  relations  are  relatively  favorable.  The  leaves  are 
large  and  the  plant  has  the  appearance  of  being  ill-adapted  to  successfully 
withstand  very  severe  arid  conditions.  The  root-system  is  not  an  extensive 
one,  and  is  characterized  byhavingamain  root  and  lateralsof  the  first  order 
of  about  the  same  diameter  and  by  the  relative  coarseness  of  all  the  roots. 
The  main  root  does  not  go  straight  down,  but  zigzags  downward  in  a  very 
irregular  fashion.  From  the  main  root  there  arise,  at  intervals  of  about  a 
centimeter,  rather  coarse  laterals  which  branch  sparsely.  It  seems  prob- 
able, although  not  proved  by  actual  experiments,  that  the  restricted  dis- 
tribution of  Boivlesia  may  be  directly  associated  with  the  shallowly  placed 
and  poorly  developed  root-system  of  the  plant. 

Brodi^a  Capitata. 

Brodicea  is  restricted  in  its  distribution  to  Tumamoc  Hill  and  to  the  upper 
bajada  slope,  and  does  not  occur  in  the  other  habitats,  namely,  Santa  Cruz 
flood-plain  or  that  by  West  Wash,  or  on  the  bajada  as  a  whole.  It  is  a 
bulbous  perennial  with  the  vegetative  and  flowering  periods  in  the  winter 
season.  Its  bulbs  are  somewhat  over  i  cm.  in  diameter,  and  are  5  cm., 
more  or  less,  beneath  the  surface  of  the  ground .  The  roots  are  adventitious, 
coarse,  and  unbranched.  They  are  relatively  few  and  are  usually  more 
than  5  cm.  in  length.  The  depth  to  which  the  roots  of  the  plant  penetrate, 
therefore,  is  approximately  10  cm. 


32  ROOT   HABITS    OF   DESERT   PLANTS. 

No  forms  of  roots  other  than  that  just  sketched  were  seen  in  the  field,  but 
dimorphic  roots  appeared  in  greenhouse  cultures.  Although  this  will  be 
referred  to  later,  it  will  be  well  to  describe  these  roots  in  this  place.  The 
conditions  under  which  the  unusual  form  of  roots  occurred  were  as  follows : 
Among  preliminary  experiments,  looking  to  a  study  of  the  aeration  of 
roots,  was  one  which  was  arranged  with  two  kinds  of  soils,  namely,  sand 
from  a  wash  and  fine  adobe  clay  (sifted)  from  the  river  bottoms.  The  bulbs 
were  planted,  or  were  intended  to  be  planted,  on  the  line,  which  was  a  verti- 
cal one,  separating  the  two  kinds  of  soil,  but  on  removing  the  plants  at  the 
end  of  the  experiment  one  bulb  was  found  to  have  been  planted  in  sand 
and  the  other  in  adobe,  about  i  cm.  in  each  instance  from  the  opposite  kind 
of  soil.  The  culture  was  running  during  a  portion  of  November  (1908) 
and  all  of  December,  which  was  a  rather  cold  period  and  unusually  cloudy. 
The  green-house  was  not  heated. 

When  removed  from  the  soil,  the  plant  of  the  adobe  side  of  the  culture 
had  two  leaves,  25  cm.  long.  The  bulb  had  nearly  or  entirely  disappeared 
and  in  its  place  was  the  fleshy  base  of  a  stout  root  from  the  upper  end  of 
which,  or  from  the  portion  of  the  bulb  remaining,  was  springing  the  usual 
type  of  roots.  The  relation  of  the  two  is  shown  in  plate  23.  Of  the 
ordinary  type  of  roots  there  were  13,  which  ranged  from  2  to  11  cm.  in 
length  and  were  wholly  confined  to  the  adobe  side  of  the  culture.  The 
central  fleshy  root  was  about  10  cm.  long.  It  was  i.i  cm. in  diameter  at  its 
crown  and  narrowed  rapidly  as  it  approached  the  tip,  which  was  attenuated. 
The  position  occupied  in  the  soil  by  the  root  was  peculiar.  In  place  of 
going  straight  down — as  did  the  root  of  the  sand-grown  plant — or  of  taking 
a  horizontal  position,  like  the  other  secondary  roots,  it  went  downward  but 
at  the  same  time  inclined  sharply  away  from  the  source  of  water  supply, 
and  ended  in  a  curl  as  shown  by  a  figure  of  the  plate. 

The  plant  on  the  sand  side  of  the  culture  in  certain  particulars  behaved  in 
a  manner  somewhat  different  from  that  described  for  its  adobe-grown  fellow. 
The  2  leaves  were  22.5  cm.  long.  The  bulb  was  not  resorbed,  and  from 
it  there  sprang  two  sorts  of  roots,  the  relations  of  which  are  indicated  in 
plate  23.  There  were  7  rather  coarse  absorption  roots,  from  i  to  7  cm. 
long,  and  a  single  fleshy  root.  All  were  of  secondary  origin ;  the  fleshy  root, 
however,  as  in  the  case  of  the  plant  described  above,  sprang  from  a  point 
on  the  bulb  very  near  its  base  and  went  straight  down  from  the  bulb ;  it 
was  7.2  cm.  long  and  0.4  cm.  in  diameter  at  the  crown.  None  of  the  roots 
entered  the  adobe  side  of  the  culture. 

An  explanation  of  the  transformation  of  fibrous  into  fleshy  roots  of 
Brodicea  will  not  be  attempted  in  this  place.  It  may,  however,  be  associated 
with  the  unusually  large  amount  of  water  available  to  the  two  plants  rather 
than  to  unfavorable  conditions  of  aeration.  A  parallel  instance,  referred  to 
later,  was  observed  in  cultures  of  several  arborescent  cacti,  inwhich  the  for- 
mation of  tuberous  roots  in  species  which  do  not  normally  produce  tuberous 
roots  was  induced  in  cultures  conducted  much  as  that  just  described. 


a 


a.  Trianthema  portulacastrum,   flood-plain  of  Santa  Cruz.     b.  Vicia  sp. 
c.  Amsinckia  spectabilis.     d.   Dauctis  pusillus. 


a.  Festuca  octoflora,  March,  1908.  b.  Gilia  bigelowii,  March,  1907.  c.  Malva 
borealis,  showing  development  of  root-system,  February,  1907.  d.  Medicago 
denticulata,  February,  1907.     e.   Microseris  liuearifolia,  March,  1907. 


ROOT   HABITS   OF    DKSKRT    PLANTS.  33 

Daucus  Pusillus. 

Dauciis  pusillus, one  of  the  most  generally  distributed  of  the  winter  annu- 
als, is  abundant  on  both  Tumamoc  Hill  and  the  upper  portion  of  the  bajada 
north  of  the  Hill.  Its  root-system  can  be  characterized  as  being  extensive 
rather  than  intensive.  The  main  root  dominates  the  root-system,  although 
it  is  never  fleshy,  or  even  very  coarse,  and  the  laterals  are  relatively  few 
and  branch  but  little. 

Young  as  well  as  mature  plants  were  studied.  The  tap  root  of  a  plant 
whose  shoot  was  i8  cm.  long  and  which  was  in  flower  was  over  15  cm.  long. 
Just  beneath  the  surface  of  the  ground,  where  the  tap  root  was  slightly 
over  I  mm.  in  diameter,  several  fine  roots,  but  not  filamentous,  were  given 
off  about  3  cm.  from  the  surface  of  the  ground,  and  3  to  4  cm.  deeper  a  prom- 
inent lateral,  7  cm.  in  length,  had  its  origin.  Roots  of  the  second  order  were 
few  in  number.  As  plate  5  shows,  there  is  no  great  difference  in  diameter 
between  the  main  root  and  the  laterals,  or  between  the  laterals  of  the  first 
and  the  second  order. 

The  leading  feature  which  marks  the  mature  plant  is  characteristic  of  the 
young  plant  as  well.  Thus  a  seedling  with  seed  leaves  and  only  one  leaf  of 
the  adult  type  had  an  unbranched  tap  root  over  5  cm.  long,  while  another 
of  about  the  same  age  had  a  tap  root  over  6  cm.  in  length,  which  every  3  to 
12  mm.  bore  laterals  about  2  mm.  long.  This  type  of  root-system  was 
seen  also  in  a  somewhat  older  plant.  It  may  be  concluded,  therefore,  that 
Daucus  not  only  penetrates  the  ground  deeply  (how  deep  was  not  deter- 
mined), but  also  quickly,  and  is  thus  admirably  adapted  to  endure  after 
more  shallow-rooted  annuals  have  perished. 

Eritrichium  Pterocaryum  and  Harpagonella  Palmeri. 

EritrichiumandHarpagonella,  two boraginaceous  winter  annuals,  although 
relatively  small,  usually  considerably  under  15  cm.  in  height,  and  thus 
inconspicuous  as  to  individuals,  occur  in  such  great  numbers  that  they 
constitute  an  important  element  in  the  plant  covering  of  their  favorite 
habitats.  These  are  common  on  Tumamoc  Hill  as  well  as  on  the  upper 
portion  of  the  bajada  to  the  north  of  the  Hill.  Plants  of  different  ages 
and  from  different  localities  were  examined.  A  mature  specimen  of  Eri- 
trichium, with  the  shoot  13  cm.  long,  had  a  filamentous  tap  root  which 
penetrated  over  8  cm.  The  laterals  were  borne  in  the  region  between  3  and 
6  cm.  from  the  surface  of  the  ground,  and  were  4.5  cm.,  and  less,  in  length. 

The  root-system  of  Harpagonell a  was  somewhat  better  developed ;  plants 
from  dry  and  moist  situations  were  studied.  Those  from  the  drier  locations 
had  unbranched  shoots  7,9,  and  10  cm.  in  length.  The  roots  of  the  smallest 
plant  penetrated  over  1 1  cm.  The  laterals  were  given  off  from  the  main 
root  from  i  to  4  cm.  beneath  the  surface  of  the  ground.  The  specimen  of 
Harpagonella  from  the  more  moist  soil,  with  a  shoot  19  cm.  long,  had  a  tap 
root  which  was  followed  over  15  cm.  The  laterals,  like  those  of  the  plant 
from  the  drier  situation,  were  unbranched. 


34  root  habits  of  desert  plants. 

Erodium  Cicutarium. 
Erodiiim  ciciitan'iiiu,  which  is  an  introduced  species,  has  already  a  wider 
distribution  in  this  vicinity  than  most  native  plants  and  promises  to  become 
one  of  the  most  successful  of  the  winter  annuals  (Spalding,  Distribution  and 
Movements  of  Desert  Plants,  Carnegie  Institution  of  Washington,  Publi- 
cation 113,  1909,  p.  27).  Although  its  root-system  was  not  studied  closely, 
there  are  some  points  of  interest  that  may  be  referred  to.  The  dominant 
features  of  the  root-system  are  the  prominent  tap  root  and  the  poor  devel- 
opment of  the  laterals.  How  deep  the  tap  root  penetrated  was  not  accu- 
rately learned,  but  its  length  is  believed  to  equal  that  attained  b}'-  the  roots 
of  most  perennials  occupying  the  same  habitats.  The  laterals  were  most 
numerous  about  4  cm.  beneath  the  surface  of  the  soil,  and  in  the  specimens 
examined  averaged  only  1.7  cm.  in  length. 

Festuca  Octoflora  and  Hordeum  Murinum. 

Two  biologic  types  of  grasses,  annuals  and  perennials,  occur  on  Tumamoc, 
each  of  which  appears  to  have  its  characteristic  distribution.  The  annual 
grasses,  both  those  of  summer  and  those  of  winter,  are  generally  distributed, 
but  the  perennial  forms  occur  where  the  water  relations  are  especially 
favorable.  Of  the  grasses  only  two  winter-growing  ones  were  examined. 
The  type  of  the  root-system  of  the  grasses  is  well  known  so  that  there  is 
no  need  of  presenting  a  description  of  it  in  detail ;  but  it  will  be  instructive 
to  compare  the  extent  of  the  root-system  of  the  grasses  with  that  of  the  other 
annuals  w^hich  appear  at  the  same  season. 

In  the  mature  plants  the  roots  are  entirely  of  secondary  origin,  and 
there  is  no  main  root  to  dominate  the  system.  The  length  of  the  roots 
is  variable  but  considerable.  In  a  plant  with  a  shoot  17  cm.  long  there 
were  3  large  roots,  each  about  5  cm.  in  length,  with  numerous  branches  2 
cm.  in  length,  more  or  less.  Another  plant  (both  were  of  the  genus  Festuca) 
19cm.  high  had  4  roots  over  12  cm.  long.  The  laterals  borne  on  these  roots 
were  filamentous  and  varied  greatly  in  length  (some  being  10  cm.  or  more), 
and  bore  in  turn  long  and  short  filamentous  branches  (plate  6) . 

The  root  system  of  Hordeum  was  similar  in  character  and  approached  in 
extent  that  of  Festuca.  A  plant  with  a  shoot  22  cm.  in  length  had  4  large 
roots,  all  of  which  were  over  14  cm.  long;  and  another  specimen  with  a  shoot 
30  cm.  long  had  7  large  roots,  of  which  the  longest  was  over  17  cm.  All 
bore  numerous  filamentous  branches.  The  depth  of  penetration  of  the 
laterals  w^as  not  learned.  It  may  be  safely  assumed,  however,  from  other 
observations  as  well  as  from  those  just  presented,  that  the  roots  reach  at 
least  15  cm.;  perhaps  still  deeper  in  the  largest  specimens. 

GiLIA    BiGELOWII. 

Gilia  bigclowii,  one  of  the  smallest  winter  annuals,  is  generally  distributed 
on  Tumamoc  Hill.  The  roots  of  Gilia  are  admirably  adapted  to  enable  the 
plant  to  reach  relatively  deep,  while  at  the  same  time  it  takes  advantage 


ROOT  HABITS  OF  DESERT  PLANTS.  35 

of  available  moisture  near  the  surface  of  the  ground.  The  leading  feature 
of  the  root-system  is  the  long  and  slender  tap  root  which  carries  numerous 
rather  short  filamentous  laterals.  The  general  character  of  the  root-system 
is  shown  in  plate  6.  The  length  of  the  tap  root  varies  with  the  age  of  the 
plant  from  5  cm.  in  one  4  cm.  high  to  over  9.5  cm.  in  a  plant  with  a  shoot  8.5 
cm.  long.  The  laterals  of  the  first  order  are  usually  less  than  2  cm.  long, 
and  branch  infrequently,  so  that  as  a  whole  the  root-system  corresponds 
very  well  with  the  "extensive"  type  of  Biisgen*  (plate  6). 

AIalva  Borealis. 

Malva,  one  of  the  most  common  of  the  winter  annuals,  was  studied  on 
Tumamoc  Hill  only,  although  it  occurs  on  the  flood-plain  of  the  Santa  Cruz 
also,  where  it  is  perhaps  most  abundant  along  the  irrigating  ditches.  The 
root-system  of  the  mature  plant  is  characterized  by  the  prominence  of  the 
laterals,  which  are  coarse  and  relatively  long.  Several  characteristic  changes 
should  be  noted,  leading  to  the  formation  of  the  mature  root-system  as 
shown  in  plate  6. 

In  the  development  of  the  root-system  of  Malva  there  are  two  fairly  well 
differentiated  stages,  of  which  the  immature  is  very  different  from  the 
mature.  The  seedling  plant  sends  down  a  tap  root  which  penetrates  the 
soil  to  a  depth  of  about  8  cm.  before  laterals  become  prominent,  or  indeed 
appear  in  numbers.  The  prominent  tap  root,  therefore,  is  the  leading 
feature  of  the  seedling.  As  the  plant  increases  in  size  the  tap  root  strikes 
down  quickly  and  strongl}^  until  a  condition  shown  in  plate  6  is  reached. 
After  this  the  laterals  begin  to  grow  and  from  this  time  they  constitute  the 
chief  feature  of  the  system.  At  first  the  laterals  arise  singly,  and  close  to 
the  surface  of  the  ground,  but  in  time  they  become  of  importance  by  their 
increase  in  length,  by  the  formation  of  branch  roots  of  the  second  order,  and 
by  the  formation  secondarily  of  laterals  of  the  first  order  along  the  main  root 
axis.  But  the  tap  root  is  always  an  important  feature,  since  in  the  mature 
plant  it  may  attain  a  length  exceeding  i6  cm. 

The  secondary  formation  of  laterals  of  the  first  order,  by  which  groups  of 
three  or  more  roots  are  formed,  which  are  of  unequal  age,  has  also  been 
noticed  in  several  other  annuals  and  probably  also  occurs  in  many  peren- 
nials. This  not  uncommon  character  of  plants  must  greatly  increase  the 
cfficiencA^  of  the  root-system  by  adding  much  to  the  absorbing  surface 
without  greatly  increasing  the  distance  of  water  transport  or  of  bringing  the 
roots  into  more  active  competition  with  those  of  neighboring  forms. 

Medicago  Denticulata. 

MedJcago,  one  of  the  introduced  forms  which  have  their  period  of  greatest 
activity  in  winter,  is  abundant  on  the  flood-plain  of  the  Santa  Cruz.  The 
plant  examined,  however,  was  growing  on  Tumamoc  Hill  in  the  vicinity  of 
the  Desert  Laboratory.     Medicago  has  a  generalized  type  of  root-system, 

*Conipare  von  Alten,  Wurzelstudien,  Bot.  Zeit.,  vol.  67,  p.  192,  1909. 


36  ROOT   HABITS    OF    DESERT   PLANTS. 

characterized  by  few  roots  which  are  rather  coarse  and  which  branch 
infrequently.  In  the  young  plant  the  tap  root  predominates,  but  as  it 
matures  the  laterals  become  important ;  the  behavior  of  the  root-system  in 
its  development  recalls  that  of  Malva.  Tubercles  were  not  present  on 
the  plants  examined.  Plate  6,  sHghtly  less  than  life-size,  shows  the  main 
features  of  the  root-system  so  that  further  description  in  this  place  is  not 
necessary. 

Mentzelia  Albicaulis. 
Mentzelia  albicaulis  occurs  on  Tumamoc  Hill  and  on  the  upper  bajada- 
slopes.  The  leading  character  of  its  root-system  is  the  prominent  tap  root. 
The  root-system  of  the  plant  was  studied  mainly  in  young  individuals,  and 
the  penetration  of  the  roots  of  the  fully  developed  plant  was  not  learned. 
A  young  plant  of  Mentzelia  with  a  shoot  2.3  cm.  in  length  had  a  tap  root 
1 1 .5  cm.  long.  The  laterals  in  young  plants  are  numerous,  unbranched  and 
occur  singly.  In  older  plants  the  laterals  are  in  groups,  as  in  Malva  and 
other  annuals. 

MiCROSERIS    TiNEARIFOLIA. 

Microseris  is  abundant  on  Tumamoc  Hill  and  on  the  slopes  of  the  bajada 
to  the  north.  The  specimens  examined  were  growing  near  the  Desert 
Laboratory  on  Tumamoc  Hill.  The  root-system,  as  shown  in  plate  6,  is 
characterized  by  the  prominence  of  the  tap  root  and  by  the  slenderness  of 
the  laterals  of  the  first  order.  The  root-system  of  a  mature  plant,  with  a 
shoot  20  cm.  in  length,  consisted  of  a  tap  root,  2  mm.  in  diameter  at  the 
crown  and  over  11  cm.  long,  and  a  few  filamentous  laterals  which  were 
borne  for  the  most  part  within  5  cm.  of  the  surface  of  the  ground.  The 
laterals  branched  sparingly.  Microseris  is  among  the  most  deeply  rooted 
of  the  winter  annuals. 

MoNOLEPis  Chenopodioides. 

Monolepis  chenopodioides  was  collected  for  study  in  February  from  the 
immediate  vicinity  of  the  Desert  Laboratory  where  it  was  growing  in 
abundance.  Its  tap  root,  as  plate  7  shows,  is  the  most  prominent  feature 
of  its  root-system,  although  laterals  are  early  developed.  The  laterals  of 
the  first  order,  although  small  as  compared  with  the  tap  root,  are,  however, 
rather  coarse,  and  branch  but  little.  Thus  the  root-system  of  Monolepis 
should  be  classified  as  extensive. 

Orthocarpus  Purpurascens. 

Orthocarpus  occurs  on  the  upper  slopes  of  the  baia  'a  especially,  and 
only  appears  in  abundance  if,  as  happened  in  the  spring  of  1908,  when  the 
root-system  of  the  annual  was  studied,  the  rains  have  been  copious.  It  is 
a  low  annual,  seldom  above  15  cm.  in  height,  of  a  gray-green  color,  and  has 
lately  been  found  to  be  an  habitual  root  parasite.* 

*Cannon:  The  Parasitism  of  Orthocarpus  purpurascms  Benth.,  Plant  World,  vol.  11, 
1909,  p.  259. 


V' 


v-\ 


...  :%,^ 


a.  Monolepis  chenopodioid.es,  February,  1907.  b.  Orthocarpus  purpurascens 
parasitic  on  Astragalus  sp.  c.  Orthocarpus  purpurascens  on  Lupinus  sp.,  March, 
1907.  d.  Parietaria  debilis,  mature  plants  from  favorable  and  unfavorable  moisture 
conditions,  e.  Phacelia  tanacetifolia,  March,  1907.  f.  Plantago  fastigiata,  March, 
1906. 


b     . 


a.   Sisymbrium  canescens.     b.   Streptanthus  californicus. 


ROOT  HABITS  OF  DESKRT  PLANTS.  37 

Parasitism  of  the  plant  has  been  observed  on  the  foHowing  species : 

Astragalus  nuttallianiis.  Eschscholtzia  mexicana.  Pectocarya  linearis. 

Bigelowia  hartwegii.  Festuca  octoflora.  Phacelia  tanacetifolium. 

Bowlesia  lobata.  Gilia  bigelowii.  Plantago  fastigiata. 

Daucus  pusillus.  Lesquerella  gordoni.  Silene  antirrhinum. 

Delphinium  scaposum.  Lupinus  sp.  Streptanthus  californicus. 

Eritrichiumpterocaryum.  Mentzelia  albicaulis.  Sisymbrium  canescens. 

The  typical  root-system  of  Orthocarpus  may  be  described  as  consisting 
of  well-developed  tap  root  and  few  laterals,  but  greater  variation  in  the 
character  of  the  root-system  of  this  species  was  seen  than  was  observed 
in  that  of  any  other  annual.  Three  leading  modifications  of  the  roots 
were  seen.  These  were:  (i)  roots  with  short  and  coarse  laterals;  (2)  those 
in  which  the  laterals  were  filamentous  and  either  long  or  short;  (3)  root- 
systems  in  which  the  tap  root  is  broken  up  into  three  or  more  forks. 

While  the  immediate  causes  of  these  variations  were  not  investigated, 
it  is  thought  that  they  may  be  connected  with  the  nature  of  the  root- 
system  of  the  host  plant,  as  will  seem  probable  from  the  following. 

In  association  with  Astragalus,  Orthocarpus  has  a  prominent  tap  root 
with  both  long  and  short  laterals  which  are  rather  coarse.  The  long  lat- 
erals are  independent  of  the  host  plant,  but  the  short  ones  bear  haustoria. 
In  the  case  of  the  length  of  the  laterals,  it  is  presumably  directly  dependent 
on  the  distance  by  which  the  parasite  and  the  host  are  separated,  which, 
however,  would  not  account  for  the  coarseness  of  the  roots  of  the  parasite. 

When  growing  in  association  with  Festuca  octoflora  as  the  host,  the  roots 
of  Orthocarpus  are  filamentous  and  the  root-system  is  poorly  developed. 
On  Eritrichium  the  roots  of  the  parasite  are  also  filamentous,  but  very 
numerous.  In  connection  with  Silene,  the  main  root  of  Orthocarpus  is 
broken  up  into  several  relatively  large  roots  and  the  laterals  are  very  few. 

Whatever  may  be  found  to  be  the  chief  causes  in  bringing  about  the 
modifications  in  the  character  of  the  root-system  of  Orthocarpus,  as  described 
above,  it  is  of  interest  to  note  that  the  soil  conditions  were  as  uniform  as 
one  would  expect  to  find  in  a  single  habitat,  and  that  the  root-systems  of 
the  host  plants  had  specific  differences  which  the  roots  of  the  parasite  to  a 
degree  reflected.  Thus  the  roots  of  Astragalus  are  coarse  and  those  of 
Eritrichium  are  filamentous,  and  the  roots  of  Orthocarpus  when  attached  to 
these  plants  are  coarse  or  fine,  as  the  case  may  be.  Whether  Orthocarpus 
can  live  independently  is  not  known,  so  what  its  root-system  would  be 
as  an  independent  plant  can  not  at  present  be  told.  However  greatly 
Orthocarpus  changes  the  character  of  its  root-system  with  differences  in 
the  roots  of  its  hosts,  it  does  not  modify  its  life-cycle  if  that  of  the  host 
chances  not  to  coincide  with  that  of  its  own.  When  associated  with 
annuals,  the  plant  would  naturally  not  be  influenced  to  prolong  its  period 
of  existence,  and  when  connected  with  a  perennial,  as  with  Bigelowia,  it 
appears  not  to  do  so  (plate  7). 


38  root  habits  of  desert  plants. 

Parietaria  Debilis. 
Parietaria  occurs  almost  exclusively  in  the  shade  of  rocks,  or  elsewhere 
where  it  enjoys  protection  either  against  excessive  light  or  severe  aridity. 
The  plant  closely  resembles  the  eastern  clearweed  (Spalding:  Distribution 
and  Movements  of  Desert  Plants,  Carnegie  Institution  of  Washington, 
Publication  113,  1909,  p.  20)  and  has  a  very  high  water  content;  though 
growing  in  the  desert  it  has  no  characteristics  which  ally  it  to  typical  desert 
plants.  Parietaria  varies  greatly  in  size  and  appearance,  a  feature  depend- 
ing on  water  relations  more  than  on  any  other  environmental  condition. 
This  may  be  illustrated  by  the  following  observation.  In  March,  1906, 
many  annuals  were  seen  to  be  of  very  unequal  size ;  the  difference  was  so 
great  in  many  instances  as  to  make  sure  recognition  of  well-known  species 
difficult.  Among  forms  exhibiting  this  condition,  none  showed  so  extreme 
modification  2iS  Parietaria,  mature  specimens  of  which,  growing  under  favor- 
able and  unfavorable  conditions  as  regards  water  relations  or  protection  by 
neighboring  plants  or  rocks,  w^ere  found  w^hose  shoots  were  from  8  mm.  to 

39  cm.  long;  that  is,  there  w^as  a  difference  of  i  to  49  in  stature  (plate  7). 
The  root-system  of  Parietaria  is  an  extremely  meager  one.     It  does  not 

penetrate  far  either  horizontally  or  vertically.  The  main  root  of  the 
plant  with  the  longest  shoot  seen  was  over  4.3  cm.,  or  about  one-ninth  the 
length  of  the  shoot.  The  length  of  the  main  root  of  the  smallest  specimens 
was  about  5  times  the  length  of  the  shoot.  The  relation  of  the  length  of 
root  and  of  shoot  in  these  instances,  together  with  the  general  character  of 
the  root-system,  is  shown  in  plate  7. 

Phacelia  Tanacetifolia. 
Phacelia  is  one  of  the  most  common  of  the  winter  annuals  on  Tumamoc 
Hill,  and  one  of  the  most  generally  distributed,  although  it  may  be  most 
abundant  and  attain  greatest  development  on  the  northern  slope  and  in 
the  protection  of  larger  plants  and  of  rocks.  A  prominent  tap  root  is  the 
chief  characteristic  of  its  root-system.  A  mature  plant  with  a  shoot  15 
cm.  high  had  a  tap  root  2  mm.  in  diameter  at  the  crown  and  over  18  cm. 
long.  The  laterals  of  the  first  order  are  fine,  usually  short,  and  arise  singly. 
Under  certain  circumstances,  however,  the  laterals  become  a  more  pro- 
nounced feature  of  the  root-system,  as,  in  a  younger  plant  than  that  above 
described,  laterals  6  cm.  in  length  were  seen  within  3  cm.  of  the  surface  of 
the  ground.  With  its  prominent  tap  root  and  fine  laterals,  Phacelia  is  to 
be  classed  among  the  plants  with  an  extensive  root-system,  like  the  most 
of  the  annuals  examined  (plate  7.) 

Plantago  Fastigiata. 

Plantago  is  abundant  on  Tumamoc  Hill  and  on  the  upper  bajada  at  its 
northern  base.  It  grows  everywhere  in  the  open  where  it  is  exposed  to  the 
sun  and  is  among  the  most  drought-resistant  of  the  winter  annuals.     The 


ROOT    HABITS    OF    DKSERT    PLANTS. 


39 


leading  characteristics  of  its  root-s}stem  are  the  long  tap  root  and  the  small 
development  of  laterals.  The  tap  root  of  the  young  plant  may  reach  as 
deep  as  13  cm.  before  laterals  appear  to  any  extent  (compare  plate  7),  and 
even  in  the  mature  form  there  are  few  laterals,  and  these  branch  but  little. 
The  extreme  depth  to  which  the  tap  root  of  the  mature  plant  reached  was 
not  learned,  but  it  may  safely  be  considered  among  the  most  deeply  placed 
of  the  roots  of  any  annual. 

Rafinksouia  Neo-mkxicana. 

Rafincsqnia  occurs  on  the  bajada  at  the  northern  base  of  Tumamoc  Hill 
and  on  the  hill  itself.  Its  root-system  is  characterized  by  a  prominent  tap 
root  which  bears  filamentous  laterals.  The  tap  root  is  2.5  mm.  in  diameter 
at  the  crown.  The  laterals  of  the  first  order  are  5  cm.  long,  more  or  less,  and 
branch  infrequently;  laterals  arise  singl}^  and  cultures  (to  be  mentioned 
below),  in  which  attempt  was  made  to  induce  the  formation  of  laterals 
secondarily  and  in  groups,  were  not  successful.  In  Rafinesqnia,  therefore, 
the  appearance  of  laterals  singly  should  be  considered  a  fixed  character. 

Streptanthus  Arizonicus,  Sisymbrium  Canescens,  and  Sisymbrium 
Reflexum. 

The  three  cruciferous  annuals  whose  root-systems  were  studied  occur 
both  on  Tumamoc  Hill  and  the  upper  reaches  of  the  bajada  at  its  northern 
base.  Streptanthus,  however,  appears  to  be  limited  to  the  Hill.  Streptan- 
thus and  Sisymhrimn  have  similar  root-systems.  The  tap  root  is  a  prom- 
inent feature,  and  the  laterals  are  fine  and  long,  and  branch  but  little.  The 
roots  thus  are  extensive  rather  than  intensive,  enabling  the  plant  to  reach 
out  as  well  as  to  penetrate  the  ground  a  considerable  distance.  Plate  S 
shows  well  the  general  character  of  the  young  and  of  the  mature  plant  of 
Streptanthus  and  does  not  need  further  comment. 

A  specimen  of  Sisymbrium  with  the  shoot  15  cm.  long  and  fruiting  was 
examined.  The  tap  root  was  relatively  heavy  and  was  traced  over  16  cm. 
into  the  soil.  The  laterals  of  the  first  order  were  either  fine  or  filamentous 
and  were  10  cm.  or  more  in  length,  and  were  borne  for  the  most  part  within 
2  to  6  cm.  of  the  surface  of  the  ground. 

Both  in  Streptanthus  and  in  Sisyinbriuni  the  laterals  arise  singly;  there 
are  no  groups  of  laterals  and  no  rudiments. 


GENERAL  CONCLUSIONS  AND  SUMMARY  OF  STUDIES  ON 
ANNUALS. 

On  Tumamoc  Hill,  and  that  portion  of  the  bajada  and  the  flood-plain  of 
the  Santa  Cruz  river  within  the  Laboratory  domain,  as  well  as  by  the  West 
Wash,  223  species  of  annuals  are  reported.  Of  these,  122  are  winter  annuals 
and  44  appear  in  summer.  Representative  species  both  of  winter  annuals 
and  of  summer  annuals,  as  well  as  one  or  two  perennials  and  long-lived 
annuals,  were  examined  in  connection  with  the  present  study.  In  looking 
over  the  results  of  observ^ations  on  root-systems,  several  facts  were  brought 
to  light,  of  which  the  following  appear  at  present  to  be  the  most  important. 

A  feature  of  several  species  of  both  winter  and  summer  annuals  is  the 
presence  of  rudimentary  roots  in  certain  species  and  their  consistent  absence 
in  others.  Among  the  plants  with  rudimentary  roots  are  Amsinckia,  Eritri- 
chium,  Harpagonella,  Pectocarya,  Erodimn,  and  Malva,  among  the  winter 
annuals,  and  Amaranthus,  Boerhaavia,  Cladothrix,  and  Trianthema  among 
those  of  summer.  Frequently  associated  with  the  presence  of  the  root 
rudiments,  perhaps  always  so,  is  the  appearance  of  laterals,  particularly 
those  of  the  first  order,  in  groups  of  three  or  four;  another  feature  is  the  pre- 
cocity of  rudimentary-root  formation  in  several  species,  particularly  in 
Amsinckia  and  its  relatives.  With  the  desire  to  throw  some  light  on  the 
causes  underlying  the  appearance  of  the  rudiments  the  following  experi- 
ments, which  are  regarded  as  preliminary,  were  set  up. 

On  February  14  two  pots  were  planted  with  seeds  oi  Amsinckia  spectabilis. 
At  the  time  of  the  sowing  of  the  seeds  the  pots  were  thoroughly  watered, 
after  which  one  was  not  given  water  again  and  the  other  was  watered 
frequently.  About  April  6  the  young  plants  in  the  dry  pot  were  seen  to 
be  in  a  wilting  condition  and  were  removed  from  the  pot  and  their  roots 
examined.  It  was  found  that  the  laterals  of  the  first  order  were  borne 
singly  on  the  main  root  and  that  at  the  base  of  each  lateral  there  were  one 
or  two  rudiments.  The  rudiments  of  the  smallest  plants  in  the  pot  were 
best  developed.  The  root-systems  of  the  plants  growing  in  the  well- watered 
pot  had  the  following  characters :  the  laterals  of  the  first  order  were  mostlv 
in  pairs,  and  at  the  base  of  each  group  there  was  at  least  one  rudimentarv 
root.  It  would  appear,  therefore,  that  the  organization  of  the  rudiments 
in  Amsinckia  is  not  dependent  on  an  improvement  of  the  water  conditions, 
although  their  subsequent  development  is,  and  that  they  are  to  be  con- 
sidered a  constant  character  in  the  root-system  of  the  species. 

In  many  of  the  plants  no  rudiments  were  seen  at  any  time ;  experiments 
were  set  up,  consequently,  to  learn  whether  the  formation  of  rudimentar}^ 
roots  might  be  induced;  the  experiments  were  confined  to  Rafiucsquia  neo- 
mcxicana  and  the  conclusions  are  not  assumed  to  be  applicable  to  anv 
other  species.  In  December  a  pot  of  suitable  earth  was  prepared  and  was 
thoroughly  soaked  with  water,  and  seeds  of  Rafincsquia  were  sown.     After 


GENERAL   CONCLUSIONS    AND   SUMMARY    OF   STUDIES.  4 1 

the  seedlings  appeared  no  more  water  was  given  the  pot  until  February  2, 
when  by  wilting  in  the  daytime  the  plants  showed  that  more  was  required. 
On  February  2  the  earth  of  the  pot  was  well  watered.  Very  shortly  after 
the  watering,  the  plants  recovered  and  began  growing  vigorously,  and  on 
February  9  they  were  removed  from  the  pot  and  their  roots  were  examined. 
It  was  learned  that  the  main  root  bore  laterals  of  the  first  order  singly ;  no 
rudimentary  roots  were  present.  It  would  therefore  appear  that  in  Rafin- 
esqiiia  not  only  are  rudimentary  roots  not  present  in  the  natural  condition, 
but  their  formation  may  not  be  induced  by  improved  water  relations. 

The  root-systems  of  the  winter  annuals  are  usually  easily  distinguishable 
from  those  of  the  summer  annuals.  The  most  striking  characteristics  of  the 
roots  of  the  former  are  the  prominently  developed  tap  root  and  the  meager 
development  of  the  laterals,  which  are  generally  filamentous  or  at  least  thin. 
The  annuals  of  summer,  on  the  other  hand,  have  root-systems  which  are 
frequently  of  a  more  generalized  form ;  that  is,  the  laterals  are  developed 
well  and  are  frequently  rather  coarse,  and  the  main  root  is  often  forked. 
The  absorbing  surface  of  the  summer  annuals  appears  to  be  greater  than  that 
of  the  winter  forms,  although  the  depth  of  penetration  of  the  roots  of  the  two 
classes  is  apparently  about  the  same. 

The  causes  leading  to  the  differentiation  in  the  root-systems  of  the  win- 
ter and  the  summer  annuals  are  not  surely  known,  but  on  the  probability 
that  they  lie  in  the  nature  of  the  species  as  well  as  in  the  difference  in  the 
environment  of  the  two  classes  of  plants  the  following  hypothesis,  as  a  basis 
for  subsequent  experimentation,  is  offered.  The  facts  in  addition  to  those 
presented  in  the  foregoing  paragraph  arc  as  follows.  The  rains  of  summer, 
in  addition  to  thoroughly  wetting  the  soil,  serve  to  cool  it  as  soon  as  they 
come.  The  air  temperature  also  immediately  falls,  and  the  relative  humid- 
ity at  once  becomes  high.  In  brief,  the  conditions  for  a  tropical  luxuri- 
ance of  growth  are  at  hand,  and  the  shoots  of  the  plants  which  appear  at 
this  season  bear  a  noticeably  large  number  of  large  leaves ;  the  transpiration 
surface  is  relatively  great. 

In  the  winter  season,  however,  the  general  character  of  the  annuals,  and 
that  of  their  environment,  are  strikingly  different  from  those  just  described 
as  obtaining  in  summer.  As  was  shown  in  the  discussion  on  soil  temper- 
atures, the  temperature  of  the  soil  begins  to  fall  with  the  coming  of  the 
summer  rains  and  continues  to  decline  until  March-April.  Therefore,  the 
rains  of  the  winter  season  do  not  materially  change  the  course  of  the  curve, 
as  is  the  case  in  summer.  The  air  temperature  at  the  time  of  the  winter 
rains  is  relatively  high,  but  much  lower  than  in  summer.  There  are  also 
occasional  periods  of  really  high  temperatures  and  drying  winds.  The 
winter  plants,  so  far  as  I  have  observed,  usually  do  not  grow  as  quickly 
and  do  not  have  as  large  leaf  surface  as  the  summer  annuals. 

Given  a  sufficient  amount  of  water  both  in  winter  and  in  summer,  it  is 
possible  that  the  difference  in  the  relative  temperature  relations  of  soil  and 


42  ROOT   HABITS    OF    DESERT   PLANTS. 

of  air  of  the  two  seasons  is  mainly  the  operating  cause  in  bringing  about  the 
difference  in  the  root-systems.  The  leading  factors  in  this  process  would  be 
as  follows.  In  summer  the  temperatures  both  of  soil  and  of  air  permit,  on 
the  one  hand,  very  rapid  growth,  and  on  the  other  the  best  conditions  for 
water-absorption,  with  the  result  that  the  root-system  of  a  plant  is  well 
developed.  In  winter,  on  the  other  hand,  the  soil  at  the  20  to  30  cm.  depth 
is  in  daytime  colder  than  the  air,  which  operates  directly  to  depress  the  rate 
of  water-absorption,  to  limit  the  development  of  the  roots,  and  thus  to  make 
the  conditions  unfavorable  for  the  fullest  growth  of  the  shoot,  with  the 
result  that  in  turn  the  demands  on  the  root-system  are  relatively  low. 
Could  the  winter  soil  be  warmer  than  the  air,  the  growth  of  the  shoot  would 
probably  be  much  more  vigorous  and  the  root-systems  of  the  plant  much 
more  extensive  and  of  a  different  character  than  they  actually  are. 

From  the  foregoing  brief  statement  of  the  leading  general  differences 
between  the  annuals  of  summer  and  those  of  winter,  it  will  appear  that  the 
summer  season  may  be  conceived  as  favoring  hygrophily  more  than  the 
winter  season ;  so  that  the  root-systems  of  the  summer  annuals  can  be  said 
to  tend  toward  hygrophily  and  those  of  the  winter  forms  toward  xerophily. 
With  these  differences  in  mind,  it  is  of  interest  to  classify  those  of  the 
winter  annuals  which  have  a  generalized  type  of  root-system,  or  one  which 
approaches  the  type  of  the  summer  plants.  As  a  fact,  we  find  that  Bowlesia 
Parietaria,  and  Malva,  the  only  plants  examined  with  a  marked  general- 
ized type  of  root-system,  are  limited  in  their  normal  distribution  to  situa- 
tions where  they  enjoy  relatively  favorable  water  relations.  The  winter 
annuals  with  specialized  or  winter  type  of  root-system,  that  is,  deeply 
penetrating,  have  the  widest  distribution. 

Although  no  attempt  has  been  made  in  this  study  to  group  plants  accord- 
ing to  the  various  types  of  root-systems,  or  to  describe  the  roots  from  the 
standpoint  of  classification,  such,  for  example,  as  has  been  done  by  Freiden- 
feldt  (Studien  iiber  die  wurzeln  krautiger  Pflanzen.  I.  Ueber  die  Formbild- 
ung  der  Wurzel  vom  biologischen  Gesichtspunkte,  Flora,  91,  1902),  it  has 
been  noted  that  each  species  examined  had  its  peculiar  form  of  roots.  So 
striking  is  the  individuality  of  the  root-systems  that  it  may  be  possible,  and 
probably  would  be  possible,  to  determine  the  form  solely  from  the  character 
of  its  root. 

The  most  striking  roots  of  autotrophic  annuals  examined  were  those  of 
Aster  ianacetifolius,  although  those  of  Amsinckia  and  its  relatives  were  also 
very  individual.  The  root-system  of  the  parasite  Orthocarpus  purpurascens, 
which  was  found  in  connection  with  18  hosts,  was  more  diversified  than  that 
of  any  other  annual.  The  roots  were  either  coarse  or  fibrous,  with  a  single 
main  root,  or  broken  up  into  several  main  roots.  Whatever  may  have  been 
the  determining  cause  of  this  diversity,  it  was  observed  that  the  roots  of  the 
parasite  reflected  in  a  degree  the  characters  of  the  roots  of  the  host  plants. 


ROOT-SYSTEMS  OF  PERENNIALS. 

The  problem  of  studying  the  roots  of  the  larger  plants  in  the  field,  and  of 
recording  the  results,  is  naturally  quite  different  from  that  of  examining  the 
root-systems  of  annuals.  The  environment  to  which  the  roots  of  perennials 
are  exposed  and  to  which  they  should  be  related,  is  likewise  much  more 
varied  than  that  of  the  smaller  plants  of  briefer  life  span.  In  the  case  of 
the  annuals  the  entire  root-system  might  by  appropriate  means  be  brought 
under  the  eye,  photographed,  measured,  and  examined  microscopically;  but 
such  procedure  is  impracticable  in  perennials,  and  choice  had  to  be  made 
arbitrarily  between  minute  examination  of  ultimate  roots,  as  Biisgen  studied 
the  roots  of  Javanese  and  German  trees,  and  a  study  of  the  more  permanent 
portions  of  the  root-systems.  Since  the  survival  means  successful  resistance 
to  desert  conditions,  and,  further,  as  the  study  was  primarily  one  in  which 
the  plant  was  to  be  compared  to  its  environment,  the  latter  method  was 
decided  on. 

Accordingly,  in  this  report  on  the  results  of  the  investigation,  the  mature 
root-systems  are  characterized  and  the  more  obvious  relations  to  tempera- 
ture and  water  are  presented. 

The  perennials  selected  for  study  comprise  the  most  striking  ones  only, 
and  probably  do  not  include  many  which  will  be  found  worthy  of  subsequent 
examination.  The  list  includes  trees  and  shrubs,  mainly  the  latter,  and 
among  these  is  a  great  diversity  in  habits  and  habitats.  There  are  fleshy 
as  well  as  non-fleshy  forms;  plants  with  shallow  and  those  with  deeply 
placed  roots ;  plants  deciduous  as  well  as  evergreen ;  those  which  come  into 
leaf  in  summer  only  and  those  which  form  leaves  whenever  there  is  adequate 
amount  of  water  whatever  may  be  the  time  of  the  year,  and  finally  there 
is  one  parasite  (Krameria). 

Following  is  a  list  of  the  species  examined,  together  with  the  habitats  in 
which  they  were  studied : 

TuMAMOC  Hill:  Echinocacius  wislizetii  Engelm.,  Encelia  fariiwsa  Gray,  Jairopha  carJio- 

phyila  (Torr.)  Muell.  Arg.,  Opnntia  discata  Griffiths,  Opuntia  Icptocaulis  DC, 

Opuntia  versicolor  Engelm. 
The  Bajada:  Camegiea  gigantea  (Engelm.)  Britt.  and  Rose,  Covilica  iridentata  (DC) 

Vail,    Fouquieria    splendens   Engelm.,    Franseria    deltoidea    Torr.,    Krameria 

canescens  Gray,  Krameria  glandulosa  Rose,  Opuntia  arbuscula  (?)  Engelm., 

Opuntia  fulgida  Engelm.,  Riddellia  cooperi  Gray. 
The   Bajada,    Rincon    Mountains:    Dasylirion  texanum  vScheele,  Mortonia  scabnlla 

Gray,  Yucca  sp..  Yucca  radiosa  Engelm. 
Flood-Plain  of  Santa  Cruz  and  West  Wash:  Condalia  spathidafa  Gray,  Ephedra 

irifurca  Torr.,  Koerberlinia  spinosa  Zucc,  Opuntia  vivipara  Rose,  Zizyphus 

parryi  Torr. 

Besides  these  forms  some  observations  were  made  also  on  Prosopis 
velutina  Wooton,  Lycium  sp.,  Olneya  tesota  Gray,  Parkinsonia  microphylla 
Torr.,  and  a  few  other  forms  which  will  be  given  incidentally  in  the  account 
of  the  root-systems  below. 


44 


ROOT   HABITS    OF    DUSERT   PLANTS. 


PLANTS  FROM  TUMAMOC  HILL. 
Of  the  areas  under  consideration  in  this  paper,  Tumamoc  Hill  is  the  richest 
in  the  number  of  species  of  perennials  and  the  bajada  in  annuals.  The  same 
species  frequently  occur  on  all  of  the  habitats,  but  to  this  there  are  numerous 
exceptions.  Of  the  plants  studied  on  Tumamoc  Hill  the  following  do  not 
grow  in  the  other  areas :  Jatropha  cardiophylla,  Opuntia  discata,  and  Opuntia 
versicolor.  On  the  other  hand,  some  forms  which  are  most  typical  of  Tuma- 
moc Hill,  such  as  Carnegiea  gigantea,  but  which  are  in  other  areas  also,  w^ere 
studied  where  the  soil  conditions  were  most  favorable  for  excavating,  and 
not  necessarily  on  the  Hill  exclusively. 

ECHINOCACTUS  WiSLIZENI. 
Two  specimens  of  Echinocactus  were  selected  for  special  study,  one  of 
which  was  growing  by  West  Wash  and  the  other  on  the  north  shoulder  of 
Tumamoc  Hill  about  60  meters  north  of  the  Laboratory  building. 


^r 


Fig.  2. — Root-system  of  Echinocactus  wislizeni,  horizontal  and  vertical  extension.  Intruders 
are  represented :  bv  broken  lines.  Acacia  conslricta;  bv  large  dot%,  Menodora  scabra;  and 
Opuntia  discata  by  dotted  lines. 


'K 


Root-system  of  Echinocactus  Wislizeni. 

A.  Showing  anchoring  and  absorbing  roots  in  natural  position,  from  West  Wash. 

B.  Bird's-eye  view  of  roots  from  which  overlying  soil  has  been  removed. 


a.  Root  and  shoot  habit  of  Opuntia  leptocaulis  which  was  growing  under  the 
protection  of  Acacia  constricta,  Tumamoc  Hill. 

b.  Opuntia  versicolor  showing  habit  of  plant  and  character  of  environment. 
A  small  specimen  of  Echinocactuswislizeni  is  to  be  seen  at  base  of  main  stem. 

c.  Bird's-eye  view  of  a  portion  of  the  central  part  of  the  root-system  of  the 
plant  shown  in  b,  with  the  overlying  soil  removed,  leaving  roots  as  far  as 
possible  in  their  natural  placing. 

d.  This  and  figures  on  plate  11  are  of  one  main  lateral  and  some  branches  of 
the  plant  shown  above. 


mBm%:^ 


\V. 


Ti 


2.y^-i.' 


1i     " 


PLATE  11 


.i«^;)ji*; 


Continuation  of  plate  10.     Bird's-eye  view  of  root-system  of  Opuntia  versicolor. 


ROOT-SYSTEMS    OF   PERENNIALS.  45 

The  general  soil  conditions  of  Tiiniamoc  Hill  have  already  been  described 
and  need  not  be  repeated  here.  It  will  only  be  necessary  to  add  that,  at  the 
precise  spot  where  the  specimen  of  Echinocactus  was  growing,  the  bed  rock 
was  so  deep  that  it  was  not  encountered  in  the  course  of  the  excavation  of 
the  plant's  roots.  The  roots  were  wholly  confined  to  the  upper  soil  layer 
or  the  malpais,  which  here  was  relatively  heavy.  In  West  Wash  (the  soil 
conditions  of  which  are  also  described  above)  the  soil  is  somewhat  lighter, 
owing  to  an  admixture  of  sand,  and  exceeds  2  meters  in  depth.  The  water 
and  the  temperature  relations  of  the  habitats  are  unlike,  and  as  far  as 
studied  are  also  characterized  in  the  section  above  referred  to. 

The  Echinocactus  examined  on  Tumamoc  Hill,  April,  1907,  was  38  cm. 
high  and  of  about  the  same  diameter.  As  plates  9  and  10  and  fig.  2  indicate, 
the  root-system  of  the  plant  has  a  very  meager  anchoring  portion  and  a  very 
extensive  absorbing  portion.  The  main  root  went  straight  down  20  cm. 
and  gave  off  several  short  laterals  between  its  tip  and  its  crown,  which 
latter  was  10  cm.  beneath  the  surface.  The  most  deeply  placed  of  the  short 
laterals  extended  12  cm.  away  from  the  main  root  axis  and  attained  a  depth 
of  25  cm. 

The  main  absorbing  system  consisted  of  three  laterals  of  the  first  order 
which  left  the  tap  root  at  its  crown  and  extended  in  two  directions,  namely, 
uphill  and  downhill.  The  larger  portion  was  that  which  extended  up  the 
hill.  The  uphill  portion  originated  from  a  single  lateral,  as  shown  in  the 
figure,  and  consisted  of  numerous  roots  of  the  second,  third,  and  fourth 
orders,  which  were  so  disposed  as  to  cover  the  ground  included  by  them 
fairly  thoroughly 

The  roots  of  the  main  absorbing  portion  of  the  system  were  of  a  light 
brown  color,  and  were  rope-Hke  and  tough ;  they  were  slender  throughout 
their  length.  The  depth  of  the  roots  may  be  learned  from  the  following 
measurements,  which  were  made  on  the  portion  which  was  placed  uphill 
from  the  main  axis  of  the  plant.  At  a  distance  of  40  cm.  the  lateral  was 
3  cm.  deep ;  3  meters  distance  it  was  3  cm. ;  and  at  3.5  meters  the  roots  were 
1.5  cm.  beneath  the  surface  of  the  soil.  Thus  the  roots  of  the  absorbing 
portion  of  the  system  are  all  extremely  shallow,  so  shallow  that  they  are 
often  exposed  by  the  eroding  effects  of  severe  storms. 

In  order  to  observe  the  possible  range  of  variation  of  the  root-S3^stem  of 
Echinocactus,  other  specimens  were  examined,  one  of  which  was  growing 
by  West  Wash.  The  leading  points  in  the  study  of  this  plant  may  be  sum- 
marized as  follows :  The  horizontal  system  was  sharply  differentiated  from 
the  vertical  system,  both  of  which,  particularly  the  latter,  were  densely 
branched.  The  horizontal  portion  of  the  root-system  lay  for  the  most  part 
within  5  cm.  of  the  surface.  It  was  apparent  that  the  anchoring  roots  were 
also  very  actively  engaged  in  absorption,  and  that  the  absorbing  roots  were 
relatively  not  so  important  as  the  corresponding  roots  in  the  specimen  from 
Tumamoc  Hill. 


46  ROOT   HABITS    OF    DESERT   PLANTS. 

The  relation  of  the  specimen  of  Echinocactus  from  Tumamoc  Hill  to  other 
perennials  included  within  the  radius  of  its  horizontal  roots  was  as  follows : 
One  specimen  of  Acacia  const ricta  was  placed  i  meter  from  the  main  axis  of 
the  plant ;  one  specimen  of  Op^mtia  discaia  was  slightly  farther  than  Acacia ; 
and  ID  specimens  of  Mcnodora  scabra,  a  shrubby  form,  occupied  various 
positions  nearer  than  the  other  species  named.  The  neighboring  forms, 
however,  held  very  unlike  relations  to  the  Echinocactus,  a  fact,  in  this  case, 
dependent  on  the  character  of  the  root-systems.  The  roots  of  Menodora 
penetrated  deeply  and  there  was  little  development  of  the  laterals  corre- 
sponding to  the  horizontal  or  absorbing  system  of  Echinocactus.  Only  a  few 
of  the  roots  oi  Acacia  were  encountered  while  excavating  for  the  root-system 
of  Echinocactus,  but  two  long  and  a  few  short  roots  of  Opuntia  were  seen 
to  lie  in  close  proximity  to  the  downhill  roots  of  Echinocactus.  From  the 
positions  occupied  by  the  roots  of  these  plants  it  may  be  concluded  that 
only  the  species  of  cactus  enter  into  active  competition  with  one  another 
for  water.  Competition  between  plants  whose  roots  occupy  horizons  20  to 
30  cm.  apart,  as  between  the  cacti  and  other  plants,  is  not  to  be  con- 
sidered as  direct. 

The  story  of  the  developmental  changes  which  take  place  in  the  root- 
system  of  Echinocactus  was  not  learned,  although  it  is  undoubtedly  different 
from  that  of  such  a  fleshy  form  as  the  sahuaro  (Carncgiea  gigantea)  in  which 
the  horizontal  portion  of  the  root-system  becomes  modified  secondarily  to 
meet  the  demands  of  the  plant  for  mechanical  support,  as  will  be  described 
below\  The  root-system  of  Echinocactus  is  not  suited  to  enable  the  epigeal 
portion  to  withstand  severe  lateral  strains,  as  is  the  case  with  the  sahuaro, 
for  which  reason  the  plants  are  often  found  uprooted,  and  are  usually 
easily  overthrown  by  a  well-directed  push.  It  is  a  matter  of  common 
observation  that  in  falling,  or  in  leaning,  the  plant  generally  leans  or  falls 
in  a  southerly  direction.  It  thus  may  be  called  the  compass-plant  of  the 
desert.  The  consistence  of  this  position  is  without  doubt  founded  on 
normal  developmental  changes  to  which  the  plant  is  subjected,  such  as  the 
greater  growth  of  tissues  on  either  the  north  or  the  south  side,  as  in  the 
sahuaro  (E.  S.  Spalding:  Mechanical  adjustment  of  the  sahuaro,  Cereiis 
giganteus,,  to  varying  quantities  of  stored  water.  Bull.  Torr.  Bot.  Club, 
vol.  32,  page  64,  1905),  by  which  an  asymmetrical  distribution  of  weight 
takes  place. 

Encelia  Farinosa. 

Encelia  has  a  very  decided  habitat  preference.  Not  only  is  the  species 
limited  to  Tumamoc  Hill,  but  it  is  most  abundant  on  the  southern  face  of 
the  hill,  though  occurring  also  on  the  west  and  east,  and  to  a  limited  extent 
on  the  north  side.  As  the  most  t5^pical  habitat  was  not  favorable  for  exam- 
ination of  the  roots  of  the  plant,  on  account  of  the  rock  exposures  and  the 
steep  gradient,  it  was  studied  on  the  northern  slope  where  the  soil  is 
deeper  and  the  roots  more  easily  removed. 


ROOT-SYSTEMS    OF    PERENNIALS.  47 

Encelia  is  a  very  striking  shrub,  less  than  i  meter  high,  with  a  close  and 
rounded  contour.  The  ends  of  the  rather  sparingly  branched  shoots,  which 
are  of  a  yellow-green  color,  bear  large  grayish  leaves  during  the  more  moist 
seasons,  and  drop  all  but  the  terminal  small  ones  when  the  arid  conditions  set 
in.  Both  leaves  and  flowers  are  formed  whenever  the  water  relations  are 
favorable. 

At  the  place  where  the  specimen  of  Encelia  was  studied  the  malpais, 
about  15  cm.  in  thickness,  was  underlaid  by  hard  caliche  of  about  the  same 
thickness,  with  a  stratum  of  rotten  caliche  between,  and  this  in  turn  was 
resting  on  a  layer  of  sandy  malpais  of  undetermined  thickness. 

The  specimen  selected  for  study  was  90  cm.  in  height  and  was  composed 
of  12  shoots,  all  of  which  bore  large  leaves.  The  plant  was  mature  and  was 
a  very  vigorous  one.  Its  root-system  was  composed  of  a  rather  stout  tap 
root,  running  directly  downward,  and  numerous  laterals  which  arose  com- 
paratively far  from  the  surface  of  the  ground.  The  main  root  went  straight 
down  through  the  malpais  to  the  rotten  caliche,  where  it  narrowed  abruptly 
and  ran  through  the  rotten  caliche  to  the  hard  caliche,  through  a  crack  in 
which  it  made  its  way  to  the  more  deep-lying  sandy  caliche  beneath.  When 
the  latter  was  reached  it  turned  abruptly  and  took  a  horizontal  course  for  a 
distance  exceeding  50  cm.  It  was  followed  to  a  depth  of  55  cm.  beneath 
the  surface  of  the  soil. 

The  laterals  of  the  first  order,  wdth  no  noteworthy  exception,  were  given 
off  in  the  malpais  stratum,  that  is,  within  20  cm.  of  the  surface,  and  were 
placed  between  15  and  30  cm.  deep.  There  were  about  5  main  laterals; 
they  branched  but  little.  The  diameter,  length,  and  position  of  the  laterals 
may  be  learned  from  measurements  on  one  of  the  typical  roots.  The  lateral 
in  question  left  the  main  root  15  cm.  from  the  surface ;  15  cm.  distant  it  was 
6  mm.  in  diameter;  at  a  distance  of  60  cm.  it  was  2  mm.  in  diameter  and 
lay  24  cm.  deep;  i  meter  from  the  main  root  axis  it  was  24  cm.  deep,  and 
at  the  tip,  1.5  meters  distant, it  lay  22  cm. beneath  the  surface.  In  addition 
to  the  larger  laterals  there  are  about  6  shorter  ones  of  the  first  order  which 
are  wholly  confined  to  the  malpais. 

All  of  the  laterals  bore  groups  of  filamentous  roots,  recalling  those  of 
Aster  tanacdifoUus,  5  cm.  more  or  less  in  length,  and  about  i  cm.  apart. 
At  the  time  the  root-system  of  Encelia  was  studied,  February  11,  these 
rootlets  were  no  longer  living,  and  the  time  of  their  origin  is  not  known,  but 
is  supposed  to  be  much  later  than  the  formation  of  the  mother  laterals  that 
bear  them.  To  judge  from  their  appearance  only,*  one  w^ould  suppose 
them  to  have  been  in  part  produced  during  the  preceding  year,  probably 
summer,  and  to  constitute  an  important  element  in  the  absorbing  system  of 
the  plant.  Further  comment  on  this  type  of  roots,  which  were  observed  on 
several  species,  will  be  given  below. 

*0n  January  4,  1910,  after  rains  had  moistened  the  ground  well,  the  roots  of  Encelia 
were  seen  to  bear  young  and  growing  rootlets  of  the  kind  described  in  the  above 
paragraph. 


48  ROOT   HABITS    OI^    DESERT    PLANTS. 

Figure  3  shows  the  horizontal  and  vertical  extensions  of  the  root-system 
of  Encelia  farinosa. 

The  specimen  of  Encelia  studied  had  very  little  competition  from  its 
perennial  neighbors.  No  other  perennial  grew  within  reach  of  its  roots, 
but  the  roots  of  two  plants,  Opuntia  discata  and  Parkinsonia  microphylla, 
reached  into  or  passed  through  the  area  occupied.  The  roots  of  the  former 
were  more  shallow  than  those  of  Encelia,  while  those  of  Parkinsonia  ran 
beneath  the  roots  of  Encelia.  It  should  not  be  concluded,  however,  that 
the  plant  is  without  competitors  for  ground  water  since  during  two  seasons, 
covering  three  months  or  more,  annuals  with  deeply  penetrating  roots 
occur  in  large  numbers  and  probably  extract  more  moisture  from  the  soil 
than  Encelia  itself  does.  The  relative  thickness  of  the  3  upper  soil  layers, 
adobe,  rotten  caliche,  hard  caliche,  are  shown  diagrammatically  in  fig.  36. 


Fig.  3. — Encelia  farinosa. 

Opuntia  Discata. 

Several  types  of  flat-stemmed  opuntias  are  native  on  Tumamoc  Hill,  and 
certain  of  them,  as  far  as  the  habitats  considered  in  connection  with  the 
present  paper  are  concerned,  are  restricted  to  the  Hill.  Among  these  is 
Opuntia  discata,  a  plant  of  varying  habit,  with  prostrate  branches  when 
young,  and  with  branches  free  of  the  ground  when  older.  Two  specimens 
of  this  plant  were  selected  for  study,  one  mature  and  the  other  young. 
The  latter  was  growing  on  the  north  face  of  Tumamoc  and  the  former  on 
the  west  side.  The  soil  conditions  where  the  cacti  were  placed  were  appar- 
ently alike ;  the  upper  soil  to  a  depth  of  1 5  to  20  cm.  is  malpais,  with  rotten 
caliche  beneath,  all  lying  on  the  bed  rock. 

The  plant  from  the  northern  exposure  had  two  shoots  composed  of  two 
and  three  '  'joints  "  each.  Both  branches  were  lying  on  the  ground,  so  that 
the  main  stem  bore  none  of  their  weight.  The  root-system  consisted  of 
several  main  roots,  which  either  ran  straight  downward  from  the  base  of 
the  main  axis  of  the  plant,  or  at  an  acute  angle,  and  also  about  8  laterals, 


ROOT-SYSTEMS   OF   PERENNIALS. 


49 


which  arose  from  these  vertical  roots  and  extended  in  a  more  or  less  horizon- 
tal direction  for  various  distances  not  far  beneath  the  surface.  There  was 
no  well-defined  tap  root,  or  supporting  roots,  all  roots  functioning  mainly 
as  absorbing  roots. 


Fig.  4. — Opuntia  discala.  Horizontal  and  vertical  extension  of  root-system. 

The  main  points  are  shown  in  fig.  4,  which  represents  the  horizontal  as 
well  as  the  vertical  extension  of  the  root-system.  For  the  most  part  the 
roots  are  confined  within  a  radius  of  50  cm.  from  the  central  axis,  but  two 
laterals,  the  one  going  uphill  and  the  other  down,  were  approximately  1.5 
mm.  in  length.  The  root  reaching  southward  maintained  a  depth  of  20  cm. 
for  a  distance  of  20  cm.  and  then  ascended  rapidly  until  it  lay  at  the  10  cm. 
level,  about  which  it  remained  during  the  rest  of  its  course.  The  roots 
extending  east  were  approximately  15  cm,  deep.  Those  to  the  west  ran 
along  the  face  of  a  boulder  at  a  depth  of  15  cm.,  and  after  leaving  this  protec- 
tion quickly  rose  and  occupied  a  position  near  the  surface.  The  roots  of  the 
Opuntia  were  slender  throughout  their  course  and  branched  very  little. 

The  roots  were  seen  frequently  to  be  in  intimate  association  with  boulders 
whose  presence  and  position  greatly  affect  the  branching  and  probably 
other  habits  of  the  root-system.  Where  the  rocks  are  shallow  the  roots  run 
beneath,  but  where  they  are  as  deep  as  25  cm.,  for  example,  they  run  along- 
side. The  roots  which  are  placed  under  rocks  are  closely  appressed  to  them 
and  form  intricate  patterns  as  they  there  branch  very  densely ;  such  roots 
also  remain  functional  for  a  longer  period  than  those  of  the  same  plant  less 
advantageously  situated.  These  features  are  clearly  to  be  associated  with 
the  better  water  relations  that  are  obtained  beneath  the  rocks  which  serve 
to  retain  as  well  as  to  conduct  water  better  than  the  bare  soil.  As  illus- 
trating this  statement  the  following  observation  on  the  penetration  of  water 
may  be  noted.  Soon  after  a  rainfall  of  half  an  inch,  by  which  the  soil  would 
usually  be  moistened  to  a  depth  approximately  8  to  12  cm.,  that  underneath 


50  ROOT   HABITS   OF   DESERT   PLANTS. 

a  boulder  was  perceptibly  wet  to  a  depth  of  25  cm.,  while  less  than  i  meter 
distant  the  soil,  free  of  large  rocks,  was  air-dry  at  this  depth. 

The  root-system  of  the  older  plant,  which  grew  on  the  western  side  of 
Tumamoc  was  different  in  some  particulars  from  that  of  the  plant  just 
described.  The  shoot  of  the  plant  consisted  of  two  main  branches  and 
three  secondary  ones,  with  a  stout  central  axis,  and  the  weight  of  all  of  the 
branches  was  born  by  the  stem.  There  was  no  main  or  tap  root,  as  in 
other  cacti,  but  several  roots,  constituting  a  brush,  left  the  base  of  the  stem 
and  penetrated  the  ground  at  an  acute  angle.  Of  these  roots,  the  longest 
attained  a  depth  of  15  cm.  in  a  horizontal  direction.  From  the  central 
group  there  arose  8  leading  roots  which  formed  the  superficial  portion  of 
the  system;  these  varied  in  length  from  50  cm.  to  1.3  m.;  the  longest  were 
those  extending  uphill.  The  superficial  roots  varied  greatly  in  depth,  some 
being  as  close  to  the  surface  as  2  cm.  and  others  penetrating  as  deep  as  20 
cm.,  but  the  deeply  placed  ones  uniformly  ran  underneath  boulders,  while 
the  more  shallow  roots  were  situated  where  the  soil  was  free  from  large 
rocks.  The  bases  of  the  superficial  roots  were  somewhat  enlarged;  for 
example,  a  root  50  cm.  long  was  1.5  cm.  in  diameter  at  its  base,  but  the 
roots  as  a  whole  were  ver}^  slender.  The  enlargement  of  the  bases  of  the 
roots  probably  represents  a  regulatory  response  on  the  part  of  the  roots 
by  which  the  weight  of  the  shoot  is  borne  and  the  strains  incident  to  winds 
successfully  withstood.  Thus  roots  functioning  primarily  as  absorbing 
organs  become,  in  the  absence  of  a  well-developed  tap  root,  the  most 
important  mechanical  support  of  the  heavy  shoot. 

The  superficial  roots  of  the  cactus  favor  absorption  of  water  even  when 
the  rains  have  been  slight,  as  maybe  illustrated  by  one  or  two  instances.  On 
October  1 7  half  an  inch  of  rain  was  recorded  as  having  fallen  at  the  Desert 
Laboratory.  Within  3  days  the  joints  of  Opuntia  discata  lost  the  shriveled 
appearance  which  they  had  during  the  dry  season  and  became  plump.  On 
another  occasion  the  response  was  noted  within  24  hours  after  a  rain  of  0.54 
inch;  also,  24 hours  after  irrigating  a  certain  plant  it  showed  by  its  increased 
turgidity  that  it  had  taken  up  water.  In  none  of  these  instances  had  the 
water  penetrated  sufficiently  to  be  of  benefit  to  plants  the  absorbing  system 
of  whose  roots  was  10  cm.  below  the  surface. 

Opuntia  Leptocaulis. 

Opuntia  lepiocauHs  grows  mainly  and  was  studied  only  on  Tumamoc  Hill ; 
it  has  very  slender  shoots,  as  is  indicated  by  the  specific  name,  and  has 
certain  biological  relations,  probably  associated  with  the  character  of  the 
shoot,  which  are  of  much  interest. 

As  a  rule  the  cacti  in  the  vicinity  of  the  Desert  Laborator}'  do  not  hold 
close  and  fixed  relations  with  other  plants ;  they  grow  near  or  remote  from 
other  plants,  apparently  wholly  according  to  chance.  But  the  species 
leptocaulis  is  a  ver}^  striking  exception  to  this,  in  that  it  rarely  occurs  alone 
or  remote  from  other  and  larger  forms.     The  shoot  habits  of  Opuntia  vary 


ROOT-SYSTEMS   OF   PERENNIALS. 


51 


with  the  position  which  it  occupies  relative  to  other  plants.  When  it  is 
isolated,  the  shoot  is  rounded  and  much-branched;  where  the  plant  is  pro- 
tected by  another  plant,  the  branches  are  few  and  may  be  much  elongated, 
and  we  shall  see  that  the  roots  also  of  the  plant  vary  with  its  position. 


Fig.  5. — Root-systems  of  Opiinlia  leplocaulis.      a,  plant  growing  at  the  base  of  Acacia  con- 
stricta.     h.  plant  growing  apart  from  other  species.     Both  figures  are  of  mature  plants. 

Another  character  of  the  species  is  the  presence  of  short  branches,  shown 
in  plate  10,  which  constitute  the  chief  chlorophyll-bearing  organs.  Under 
favorable  water  conditions,  these  short  branches  may  be  very  numerous, 
but  where  the  moisture  conditions  are  otherwise,  they  are  few.  The 
branches  really  function  as  leaves,  in  that  they  are  formed  at  times  of  active 
growth  and  that  their  subsequent  history  varies  with  the  moisture  relations ; 
if  favorable,  they  are  long  retained ;  if  unfavorable,  they  early  fall  away. 
This  effect  can  be  quickly  achieved  by  uprooting  a  plant  and  hanging  it  in  a 
dry  place,  under  which  treatment  the  short  branches  to  a  large  extent  soon 
drop  off.  The  exfoliated  branches  do  not  commonly,  if  ever,  act  as  propa- 
gative  bodies,  as  is  the  case  in  other  species,  notably  in  0.  vivipara,  as  will  be 
show^n  below. 

The  root-systems  of  two  specimens  of  lepiocaiilisweve  studied  in  February, 
1907;  one  was  growing  at  the  foot  of  Acacia  constricta  (fig.  5a)  and  the 
other  independently  (fig.  56).  Both  plants  w^ere  on  the  northern  slope  of 
Tumamoc  Hill  under  the  same  conditions  of  soil  and  moisture  as  described 
above  for  Encelia.  The  protected  plant  was  80  cm.  high,  and  the  sub-aerial 
portion  consisted  of  a  central  axis  with  a  few  long  slender  branches  that 
carried  many  short  ultimate  ones  of  the  character  spoken  of  in  a  preceding 


52  ROOT   HABITS    OF   DESERT   PLANTS. 

paragraph.  The  entire  habit  of  the  plant  was  symmetrical  and  loose. 
Figure  5  and  plate  10  show  the  extension  of  the  roots  in  the  ground  and  their 
appearance  when  removed.  There  was  a  tap  root,  which  penetrated  15  cm. 
and  which  bore  a  few  laterals  not  over  10  cm.  long,  and  about  4  main  laterals 
3  of  which  extended  from  the  main  axis  as  far  as  30  cm.  and  varied  in  depth 
from  5  mm.  to  5  cm.  All  of  the  laterals  were  slender  and  bore  almost  no 
branches.     Thus  the  root-system  was  very  meagerly  developed. 

The  specimen  which  was  growing  apart  from  other  plants  was  50  cm. 
high,  of  a  compact  growth-habit,  and  was  much  branched.  The  root- 
system  of  this  plant  may  be  briefly  described  as  follows.  There  was  no  well- 
defined  tap  root,  but  there  was  a  brush  of  slender  roots  varying  much  in 
length.  The  roots  radiated  out  from  60  cm.  to  2.8  meters  and  the  longest 
was  less  than  i  cm.  in  diameter  at  its  base.  The  depth  attained  by  the 
longest  roots  ranged  from  4  cm.  to  14  cm.,  with  the  usual  depth  between 
5  and  8  cm.     The  roots  branched  hardly  at  all. 

In  contrasting  the  root-systems  of  the  two  specimens  it  will  be  seen  that 
that  of  the  protected  plant  is  composed  of  fewer  and  shorter  roots  than  that 
of  the  unprotected  one,  and  that  the  roots  of  the  former  are  also  nearer  the 
surface  of  the  soil.  In  either  instance  it  is  probable  that  the  character  of 
the  root-system,  at  least  as  far  as  regards  the  number  and  the  length  of  the 
roots,  is  immediately  connected  with  the  development  of  the  shoot,  and 
that  the  character  of  shoot  development  and  the  vertical  position  of  the 
roots  are  intimately  associated  with  the  protected  or  unprotected  condi- 
tion of  the  plant. 

Opuntia  Versicolor. 

Opuntia  versicolor,  one  of  the  cylindro-arborescent  forms,  is  one  of  the 
best  represented  species  on  Tumamoc  Hill,  although  in  no  place  does  it 
occur  in  large  enough  numbers  to  constitute  the  dominant  plant  type.  The 
habitat  where  the  plants  studied  were  growing  was  in  the  main  similar  to 
that  described  for  Encelia  farinosa,  although  it  differs  in  certain  details. 
The  upper  malpais  soil  is  relatively  deep  and  carries,  on  its  surface  and 
embedded  in  it,  stones  and  boulders  in  some  abundance.  The  water  con- 
ditions are  better  than  where  Encelia  grew,  owing  to  the  nearly  level  char- 
acter of  the  ground,  the  rather  deep  soil,  and  also  to  the  fact  that  the  place 
receives  considerable  water  as  superficial  run-off  and  as  seepage  from  higher 
portions  of  the  hill. 

Two  specimens  of  versicolor  were  studied,  of  which  one  was  young  and  the 
other  mature.  The  younger  plant  was  less  than  50  cm.  high  and  had  few 
branches.  There  were  two  well-defined  root  types  in  the  plant,  namely, 
those  shallowly  placed  and  extending  horizontally,  and  those  extending 
vertically  and  reaching  rather  deep.  The  latter,  the  anchoring  or  sup- 
porting system  primarily,  consisted  of  a  tap  root,  with  short  and  long  lat- 
erals, M^hich  penetrated  to  a  depth  of  20  cm.  One  branch  of  the  tap  root 
attained  a  depth  of  30  cm.     The  superficial  portion  of  the  root-system  was 


^ 


a,  Vertical  and,  b,  horizontal  extensions  of  root-system  of  Opuntia  versicolor,  Turaamoc 
Hill,  views  of  which  are  shown  in  plates  10  and  11.  One-fortieth  natural  size.  The  root-area 
of  the  cactus  contained  other  perennials  of  which  one  specimen  of  Echinocactus  wislizeni,  at  the 
base  of  the  cactus  studied,  is  shown  in  yellow,  one  specimen  of  Jatropha  cardiophylla  in  purple, 
and  the  tips  of  the  roots  of  a  neighboring  Opuntia  discata,  on  margin  of  plate,  are  in  yellow. 


Root-habit  of  Jatropha  cardiophylla,  of  which  the  horizontal  extension 
of  the  root-system  was  shown  in  plate  12. 


ROOT-SYSTEMS   OF   PERENNIALS.  53 

composed  of  two  laterals  which  left  the  main  root  just  beneath  the  surface  of 
the  soil  and  ran  in  opposite  directions  each  for  about  a  meter.  These  roots 
were  little  branched  and  lay  within  5  cm.  of  the  surface.  In  addition  to  the 
difference  in  the  position  of  the  two  portions  of  the  root-system  there  was  a 
further  difference  in  size :  the  deeper  part  being  uniformly  of  greater  diam- 
eter than  the  more  shallow  horizontal  portion. 

The  older  plant  whose  roots  were  examined  was  situated  about  10  meters 
south  of  the  one  just  described.  It  was  i  meter  high  and  bore  several 
branches,  and  although  mature  was  somewhat  under  the  average  size  for 
the  species.     The  habit  of  the  plant  is  shown  in  plate  10. 

In  the  immediate  neighborhood  of  the  cactus,  or  growing  within  the  reach 
of  its  roots,  were  numerous  perennials,  Echino cactus  wislizeni,  Menodora 
scabra,  Jatropha  cardiophylla,  Cassia  bauhinoides,  Acacia  constricta,  and  Par- 
kinsonia  microphyUa.  There  also  were  several  species  of  annuals.  The  posi- 
tion of  specimens  of  JMenodora  and  the  census  of  the  species  within  the 
versicolor  area  are  as  follows:  (i)  50  cm.  northeast;  (2)  1.4  meters  north; 
(3)  2  meters  north;  (4)  2.2  meters  southwest;  (5)  2.6  meters  west;  (6)  50 
cm.  south;  (7)  60  cm.  southwest;  (8)  1.5  meters  west;  (9)  1.6  meters  east; 
(10)  2.1  meters  east;  (11)  1.2  meters  southeast.  Of  these  plants,  only  one 
was  placed  so  that  the  roots  of  the  cactus  did  not  go  near  it.  The  situation 
of  Jatropha  and  Echinocactus  are  indicated  in  plate  12.  Of  other  perennials 
growing  in  the  area,  two  specimens  of  Cassia  were  2.8  meters  southeast 
and  2.2  meters  south  respectively,  one  specimen  of  Acacia  was  2.7  meters 
northwest,  and  one  Parkinsonia  was  2.2  meters  southeast.  Among  the 
herbaceous  iorxns  Astragalus  nuttaUianus ,  Daucus  pusillus,  Brodicea  capitata, 
Harpagonclla  palmeri,  and  Hilaria  sp.,  all  of  which  except  Astragalus  and 
Daucus  were  in  flower  at  the  time  the  roots  of  the  cactus  were  examined. 
No  record  was  to  be  had  of  the  number  or  kind  of  the  summer  annuals  which 
belonged  to  the  area.  In  addition  to  the  perennials  and  annuals,  both  of 
summer  and  of  winter,  which  were  growing  within  reach  of  the  cactus 
roots,  the  roots  of  a  neighboring  specimen  of  Opuntia  discata  invaded  the 
area.  Thus  within  a  space  of  little  more  than  20  square  meters  there  were 
16  perennials  belonging  to  6  genera,  and  numerous  herbaceous  forms,  of 
which  those  appearing  in  winter  belonged  to  5  genera.  This  area  was  the 
richest  in  variety  as  well  as  in  number  of  plants,  both  perennials  and 
annuals,  of  any  observ^ed  during  the  course  of  this  investigation. 

The  root-system  of  the  older  cactus  was  an  extensive  one,  the  leading 
characters  of  which  are  shown  in  plates  10  and  1 1  and  require  little  further 
description.  The  root-system  is  composed  of  a  widely  extending  horizontal 
portion  and  a  rather  deeply  penetrating  portion.  The  former  system  was 
not  seen  to  attain  a  depth  greater  than  5  cm.,  but  the  latter  reached  as 
deep  as  25  cm.  The  horizontal  roots  of  the  first  order  are  slender  except 
at  the  bases,  which,  as  the  plates  show,  are  rather  heavy.  The  laterals 
branch  but  little  except  where  they  run  underneath  stones,  where,  as  in 


54  ROOT   HABITS    OF   DESERT   PLANTS. 

Opuntia  discata,  they  are  much  branched.  Besides  the  more  prominent 
roots,  such  as  those  just  referred  to,  there  are  filamentous  ones  which  occur 
in  groups  and  which  are  usually  about  i  cm.  long.  Such  filamentous  roots 
aje  to  be  seen  throughout  the  most  of  the  course  of  the  superficial  larger 
laterals. 

Growing  within  the  compass  of  the  roots  of  the  cactus,  as  described  in  an 
earlier  paragraph,  there  were  several  perennials  whose  position  would  indi- 
cate, if  other  conditions  were  favorable,  that  they  competed  with  the  cactus 
for  water.  An  examination  of  the  roots  of  these  plants  indicated  that  this 
might  be  the  case  in  some  species,  but  that  it  probably  was  not  true  in 
others.  Menodora,  for  example,  has  a  tap  root  which  penetrates  deep 
but  has  little  extension  of  laterals,  and  the  roots  of  Parkinsonia  also  pene- 
trate the  ground  more  deeply  than  those  of  the  cactus.  These  plants,  there- 
fore, did  not  enter  into  close  competition  with  the  cactus. 

Of  the  other  aliens,  Echinocactiis  would  be  supposed  to  have  superficial 
roots  which  would  be  placed  similarly  to  the  corresponding  roots  of  versi- 
color, but  owing  to  the  fact  that  the  specimen  was  young,  the  anchoring 
roots,  and  not  the  widel}-  extending  ones,  were  developed,  and  competition 
between  the  two  species  of  cacti  had  not  yet  begun.  The  case  was  different, 
however,  with  the  specimen  oijatropha  and  the  neighboring 0/)«M^ia  discata. 
The  distal  roots  of  one  lateral  of  the  discata,  as  the  plate  indicates,  occupied 
the  same  horizon  as  those  of  versicolor,  and  were  mingled  with  the  roots  of 
the  latter  cactus.  There  must  have  been  active  competition  for  water, 
therefore,  between  the  roots  of  the  two  cacti.  The  root-system  of  Jatropha 
was  of  the  sort  that,  had  the  plant  been  more  fortunately  placed,  active 
competition  with  it  must  also  have  occurred. 

The  root-system  of  Jatropha  is  entirely  superficial  and  placed  fairly  close 
to  the  surface  of  the  ground.  As  shown  in  plates  12  and  13,  the  root  sys- 
tem of  the  plant  was  composed  of  a  long  and  fleshy  main  root,  which  ran 
horizontally,  and  several  shorter  roots,  some  of  which  were  fleshy  also. 
There  were  also  two  relatively  short  fleshy  roots  which  arose  from  the 
enlarged  shoot-base  and  ran  horizontally.  At  a  point  10  cm.  from  its  base, 
the  main  root  was  2.5  cm.  in  diameter.  The  terminal  rootlets  of  Jatropha 
were  placed  at  about  the  depth  of  the  small  superficial  roots  of  the  cactus, 
and  the  plant  would  have  competed  with  the  cactus  actively  had  the  roots 
of  the  two  species  chanced  to  occupy  the  same  territory.  Thus  it  seems 
that  despite  the  large  population  of  the  area  the  roots  of  Opuntia  versicolor 
were  in  close  relation  with  those  of  but  one  alien,  and  that  one  with  its 
central  axis  relatively  remote.  However,  this  statement  would  undoubtedly 
be  modified  if  consideration  were  given  to  the  relation  of  the  roots  of  the 
cactus  and  those  of  the  numerous  summer  and  winter  annuals  that  also 
occupy  the  area,  as  the  roots  of  many  of  them  must  be  in  close  relation,  if 
not  in  physical  contact,  with  those  of  the  cactus. 


ROOT-SYSTEMS    OF    PERKNXIALS.  55 

THE   BAJADA. 

The  plants  of  the  bajada  which  were  studied  either  were  pecuhar  to  or 
characteristic  of  the  bajada  although  not  confined  exclusively  to  that  habi- 
tat, or,  being  characteristic  of  other  formations,  occurred  nevertheless  on 
the  bajada  where  for  different  reasons  they  were  found  most  convenient  for 
study.  Covillca  tridcntata  is  found  in  greatest  numbers  on  this  habitat, 
where  it  may  in  places  produce  almost  a  pure  growth,  but  the  species  is  to 
be  found  on  the  other  areas  also  and  in  fact  attains  its  largest  development 
on  the  flood-plain  or  close  by  streamways.  Riddellia  coopcri  occurs  mainly 
on  the  bajada,  and  Opuntia  arhuscula  and  0.  fulgida  grow  only  there.  Of 
other  species  whose  root-systems  were  examined,  Carnegiea  gigantea,  Fou- 
quieria  splcndens,  Franscria  dclioidea,  Krameria  canescens,  and  K.  glandu- 
losa  are  found  on  Tumamoc  Hill  also,  and  occasionally,  on  the  flood-plain 
save  Fouquicria  Avhich  was  not  seen  on  the  last  named  formation. 

Two  species  of  Yucca,  an  Agave,  and  a  Dasylirion  from  the  bajada  to  the 
WTst  of  the  Rincon  mountains,  about  20  miles  east  of  Tucson,  were  also 
studied. 

Carnegiea  Gigantea. 

The  root-systems  of  several  specimens  of  the  sahuaro  have  lieen  observed, 
but  the  study  has  been  carried  on  mainly  on  plants  from  the  bajada  imme- 
diately west  of  West  Wash,  and  from  the  flood-plain  of  the  wash  itself.  The 
soil  conditions  comprise  very  shallow  as  well  as  fairly  deep  surface  soils,  as 
a  result  of  which  the  extreme  variability  of  the  roots  might  be  expected. 
The  water  relations,  also,  were  very  diverse,  and  probably  the  temperature 
conditions  as  well.  However,  it  should  be  stated  that  this  cactus  not  only 
avoids  the  habitats  where  the  soils  are  shallowest,  but  also  the  opposite 
extreme,  where  they  are  the  deepest;  or.  briefly,  it  does  not  occur  as  a 
general  rule  on  the  bajada  or  the  flood-plain,  but  rather  on  rocky  areas, 
chiefly  on  the  southern,  the  western,  and  the  eastern  exposures. 

The  bajada  habitat  is  at  the  eastern  end  of  the  long  slope  which  leads 
away  from  the  main  range  of  the  Tucson  mountains.  The  soil  at  the  place 
where  the  cactus  was  studied  is  of  a  dark  color,  closely  resembling  the 
malpais  of  Tumamoc  Hill,  and  is  rather  thickly  strewn  with  large  and  small 
stones.  A  section  of  the  soil  shows  the  following  leading  characters:  The 
uppermost  soil,  about  30  cm.  in  thickness,  is  of  the  adobe-malpais  referred 
to  above ;  beneath  the  superficial  stratum  is  a  thinner  one  of  caliche  which 
rests  on  bed  rock.  (See  figure  to  the  left,  plate  14.)  Numerous  stones  and 
boulders  are  embedded  both  in  the  adobe  and  in  the  caliche  layers.  The 
contour  of  the  ground  at  the  place  is  such  that  little  if  any  water  comes  to 
it  by  seepage  from  higher  ground,  or  by  superficial  run-off,  the  plants 
obtaining  all  of  the  water  supply  directly  from  the  rains. 

The  root-system  of  the  plant  is  differentiated  into  an  anchoring  portion, 
which  is  the  more  deeply  placed,  and  a  superficial  portion,  which  is  pre- 
eminently the  part  engaged  in  absorption,  although  it  has  other  functions. 


56  ROOT   HABITS    OF   DESERT   PLANTS. 

Of  the  specimens  of  sahuaro  studied,  the  first  to  be  described  was  a  healthy 
and  very  vigorous  plant  i .  2  meters  high  and  35  cm.  in  diameter  at  the  largest 
part.  The  anchoring  system  consisted  of  a  stout  tap  root  which  went 
straight  down  to  a  depth  of  30  cm.,  or  to  the  caliche,  and  a  few  laterals,  of 
which  one  ran  horizontally  about  25  cm.  The  superficial  system  arose  from 
the  main  root  close  to  the  surface  of  the  ground  and  was  composed  of  about 
6  main  laterals  which  radiated  from  the  main  axis  so  as  to  divide  fairly 
equally  the  available  area  between  them.  They  extended  from  1.5  to  5 
meters  from  the  central  axis,  and  branched  but  little.  As  an  instance  of 
the  latter  characteristic  it  may  be  stated  that  four  of  the  laterals  did  not 
give  off  branches  during  1.5  meters  of  their  course,  and  one  root,  which 
branched  freely  at  the  tip  and  was  3  meters  long,  bore  only  4  branches.  The 
superficial  roots  end  in  a  tuft  of  fine  rootlets,  nearly  all  of  which  die  with 
the  advent  of  the  drier  season,  and  groups  of  delicate  rootlets  are  borne  by 
the  superficial  roots  the  greater  part  of  their  length.  It  is  probable  that 
these  fine  rootlets  are  produced  each  year  with  the  coming  on  of  favorable 
conditions  of  growth  and  water  absorption,  as  in  Franseria  dcltoidca  and 
other  desert  plants.  The  plant  is  thus  enabled  to  exploit  thoroughly  more 
of  the  root-area  than  would  otherwise  be  possible. 

The  superficial  portions  of  the  root-system  penetrated  the  ground  to  a 
depth  which  was  fairly  uniform,  although  in  no  case  very  great.  For 
example,  one  root  leftthemain  root  at  the  surface  of  the  ground,  and  bothit 
and  its  three  main  branches,  with  little  deviation,  ran  7  cm.  from  the  surface. 
Another  root  ran  from  3  to  40  cm.  from  the  surface,  but  gave  off  a  branch 
which  went  straight  down  to  a  depth  of  1 7  cm.  Others  of  the  superficial 
system  varied  between  5  and  15  cm.  in  depth,  and  one  dipped  under  a  large 
boulder,  whose  lower  surface  was  30  cm.  from  the  general  ground-level,  after 
w^hich  it  ascended  to  about  the  depth  characteristic  of  the  older  parts. 

Where  the  larger  of  the  superficial  roots  left  the  tap  root  they  were  rela- 
tively heavy,  but  they  tapered  rapidly  until  a  small  diameter  was  reached 
which  was  maintained  with  little  change  for  a  long  distance.  A  few  meas- 
urements will  show  the  point.  One  of  the  larger  laterals  was  1.6  cm.  in 
diameter  at  its  base;  another  was  7  mm.  in  diameter;  another  root,  45  cm. 
from  its  base,  was  i  cm.  in  diameter,  and  it  extended  2.5  meters  beyond  the 
point  of  measurement.  Roots  not  above  50  cm.  long  were  about  3  mm.  in 
diameter  at  the  base.  A  root  5.5.  meters  in  length  was  2  cm.  in  diameter 
30  cm.  from  its  base. 

In  a  sahuaro  of  the  size  of  the  one  under  discussion,  it  is  probable,  there- 
fore, that  the  tap  root  constitutes  the  main  support  by  which  the  plant 
successfully  withstands  the  pressure  of  the  wind,  which  at  times  is  heavy, 
and  that  the  superficial  roots  function  mainly  as  absorbing  organs.  With 
the  growth  of  the  plant,  however,  the  need  of  anchorage  is  greatly  increased 
and  the  tap  root  no  longer  suffices  as  the  sole  stay,  so  that  the  laterals  assume 
the  added  role  of  mechanical  supports  and  become  much  changed  in  size, 
perhaps  in  other  regards  also,  to  meet  the  new  demands.     This  develop- 


Horizontal  and  vertical  extensions  of  root-systems  of  Carnegiea  gigantea,  Covillea 
tiidentata,  and  Parkinsonia  microphylla,  from  the  bajada  west  of  West  Wash, 
December,  1906.  The  Carnegiea  is  represented  by  black,  the  Covillea  by  yellow,  and 
the  Parkinsonia  by  green. 


A.  Anchoring  roots  and  bases  of  some  lateral  roots  of  Carnegiea  gigan- 
tea,  6.8  m.  high,  from  the  bajada  near  West  Wash. 

B.  Tap-root  and  secondary  roots  making  up  the  anchoring  system  and 
the  bases  of  some  of  the  superficial  roots  of  a  specimen  of  Carnegiea,  1.2  m. 
high,  from  same  habitat  as  the  cactus  shown  in  A. 

C.  Tap-root  and  bases  of  laterals  of  Parkinsonia  microphylla  which  was 
growing  near  Carnegiea  shown  in  B,  and  which  appears  in  plate  14. 


ROOT-SYSTEMS    OF    PERENNIALS.  57 

mental  modification  can  be  shown  by  the  main  characters  of  the  root-system 
of  a  larger  specimen.  A  sahuaro  6.8  meters  high  and  bearing  4  branches, 
from  40  cm.  to  i  meter  in  length,  and  growing  in  a  situation  apparently 
similar  to  that  of  the  cactus  above  described,  had  a  relatively  heavy  root- 
system.  The  roots  of  the  anchoring  system  were  sharply  differentiated 
from  the  superficial  roots  and  were  composed  of  a  cluster  of  10  main  roots 
with  their  branches  which  ran  directly  downward  about  77  cm.,  and  ended 
abruptly  on  reaching  the  bed  rock.  Of  the  superficial  system  three  leading 
laterals  were  uncovered,  one  of  which  ran  southeast  over  9.7  meters,  one 
northeast  over  4  meters,  and  one  west  over  4.5  meters.  All  of  these  roots 
were  verj-  heavy,  especially  at  the  base,  as  plate  15  indicates.  In  this,  and 
in  other  specimcms  examined,  it  is  clear  that  the  increased  diameter  of  the 
superficial  roots  is  connected  with  the  large  size  of  the  sub-aerial  portion  of 
the  plant  and  its  need  of  better  support  than  would  be  afforded  by  the 
"anchoring"  roots  alone.  A  proof  of  the  importance  of  the  laterals  as 
mechanical  supports  of  the  stem  is  often  afforded  by  the  spectacle  of 
ujDrooted  plants,  whose  laterals  have  become  diseased  and  no  longer  func- 
tion as  efficient  props  or  stays. 

A  specimen  of  sahuaro  growing  on  the  flood-plain  of  West  Wash  was  also 
examined.  The  root-system  did  not  differ  materially  from  either  of  those 
above  described ;  that  is,  there  was  an  anchoring  portion  and  a  superficial 
portion,  which  in  the  plant  seen  (which  was  less  than  2  meters  high)  were 
not  as  yet  sharply  differentiated  either  in  function  or  in  position.  The  main 
root  did  not  penetrate  deep,  although  the  soil  was  somewhat  deeper  than 
at  either  of  the  other  habitats  mentioned.  This  was  the  leading  point  of 
interest :  the  root-system  of  the  sahuaro  is  essentially  superficial  even  if  the 
soil  conditions  are  such  as  to  permit  deep  penetration. 

From  the  brief  discussion  of  the  root-system  of  the  sahuaro  it  will  be  seen 
that  there  is  no  sharp  distinction  in  function  between  the  superficial  roots 
and  the  anchoring  roots,  as  is  to  be  found  in  Echinocadus  especiall}',  but 
that  as  the  plant  increases  in  size  the  superficial  system  gradually  assumes 
the  role  of  supporting  the  sub-aerial  portion  in  addition  to  its  chief  role 
in  the  absorption  of  water. 

In  the  immediate  vicinity  of  the  cacti  growing  on  the  bajada  were  also 
the  following  perennials :  Echinocereus  fendleri,  Covillea  tridentata,  Encelia 
farinosa,  Kramcria  canesccns,  Parkinsonia  micro phylla,  and  some  other 
plants,  especially  species  of  cacti. 

The  relation  of  the  roots  of  sahuaro  to  those  of  other  perennials  was  exam- 
ined closely  only  in  connection  with  the  study  of  the  sahuaro  which  was  1.2 
meters  high.  In  this  case  one  specimen  of  Parkinsonia  and  one  of  Covillea 
were  situated  within  the  proper  root-area  of  the  sahuaro,  as  plate  14  shows. 
The  Parkinsonia  was  growing  1.3  meters  distant  from  the  stem  of  sahuaro. 
The  following  were  the  leading  points  in  the  character  of  its  roots.  There 
was  a  tap  root  which  went  straight  down  to  the  caliche,  where  it  terminated 


58  ROOT    HABITS    OF    DESERT    PLANTS. 

abruptly.  Several  slender  roots  left  the  main  root  between  5  and  10  cm. 
from  the  surface  of  the  ground  and  either  extended  fairly  horizontally  or 
dipped  downward  to  a  depth  of  45  cm.  and  even  deeper,  where  they  pene- 
trated the  caliche.  The  root-system  of  Covillca,  so  far  as  its  position  was 
concerned,  was  similar  to  that  of  Parkins onia;  that  is,  it  extended  down- 
ward through  the  malpais  and  the  caliche,  and  ran  on  the  surface  of  the 
bed  rock. 

The  relation  of  the  root-systems  of  the  three  species  is  shown  graphically 
in  the  cuts  of  the  horizontal  and  vertical  extension.  It  will  be  seen  that, 
although  growing  in  close  proximity  to  one  another  (the  Covillca  was  only 
50  cm.  from  the  cactus),  the  roots  are  not  in  physical  contact  anywhere,  so 
that  competition  for  water  in  the  soil  is  probably  not  keen  in  this  instance, 
notwithstanding  the  proximity  of  the  plants. 

CoviLivEA  Tridentata. 

Perhaps  the  most  widely  distributed  perennial  in  the  vicinity  of  the 
Desert  Laboratory  is  Covillea  tridentata,  which  occurs  not  only  on  the  bajada, 
where  it  is  the  most  characteristic  shrub,  but  on  the  flood-plain  of  the 
Santa  Cruz  river  and  West  Wash,  and  on  Tumamoc  Hill.  It  attains  its 
most  luxuriant  growth  on  the  flood-plain,  or  elsewhere  where  the  soil  is 
deep  and  the  water  conditions  relatively  favorable.  The  specimens  spe- 
cially studied  were  situated  on  the  flood-plain  of  West  Wash  and  on  the 
bajada  at  the  northern  base  of  Tumamoc  Hill.  In  these  locations  the 
extremes  of  soil  conditions,  as  far  as  concerns  its  depth,  were  met,  together 
with  the  greatest  differences  in  water  relations  to  be  found  in  the  four 
habitats  under  consideration. 

The  upper  soil  at  the  bajada  habitat  is  of  adobe  clay,  with  an  admixture 
of  rock  fragments  and  larger  stones,  about  20  cm.  in  thickness,  below  which 
is  a  layer  of  rotten  caliche,  also  about  20  cm.  thick,  and  the  latter  is  under- 
laid by  hard  caliche  of  undetermined  depth.  A  section  of  similar  soil  con- 
ditions, made  from  a  photograph  of  a  cut  not  farfrom  the  habitat  inquestion, 
is  shown  in  plate  i,  and  illustrates  the  conditions  which  obtain  here. 

The  larger  plants  of  the  vicinity  were  all  of  the  genus  Covillca,  of  which 
those  nearest  the  specimen  were  1.5  meters  northwest,  2.6  meters  southwest, 
and  3.1  meters  south.  The  roots  of  each  of  these  plants,  and  probably  the 
roots  of  others  also,  invaded  the  root-area  of  the  specimen  specially  studied. 

The  Covillea  examined  was  95  cm.  high  and  was  composed  of  numerous 
branches.  It  was  a  typical  and  vigorous  plant.  The  leading  characters 
of  its  root-system  were  found  to  be  as  follows:  A  main  root  went  down 
to  the  hard  caliche,  about  35  cm.,  where  it  forked.  One  branch  was  traced 
about  3  meters  northwest  and  lay  from  35  to  53  cm.  beneath  the  surface 
of  the  ground.  The  other  branch  was  followed  about  i  meter  and  lay  from 
35  to  45  cm.  deep.  These  were  the  longest  roots  of  the  plant  and 
extended  to  pockets  in  the  hard  caliche. 


ROOT-SYSTEMS    OF    PERENNIALS. 


59 


Fig.  6. — Root-systems  of  Covillea  tridenlala,  from  various  locations  on  the  domain  of  the 

Desert  Laboratory.  .  .  ,        r    , 

a  and  h.  Horizontal  extensions  of  root-system  of  one  plant,  separated  tor  sake  ot  clearness. 

Dotted  lines  indicate  the  intruding  roots  of  neighboring  Covilleas  encountered  on  the 

same  level,  or  nearly  so.  as  the  roots  of  the  plant  studied, 
c,  Vertical  extension  of  bajada-grown  plant.  ,     ,    .       ,  „.        „,     , 

d  and  r.  Vertical  extensions  of  root-systems  of  two  plants  from  the  flood-plam  of  Uest  \V  ash. 

Hntire  length  of  laterals  in  c  is  not  shown. 

About  ID  large  and  numerous  small  laterals  were  given  off  from  the 
main  root  within  15  cm.  of  the  surface  of  the  ground.  These  roots  were 
from  5  mm.  to  i.i  cm.  in  diameter  at  the  base,  and  the  longest  extended 
as  far  as  4  meters  from  the  central  root  axis.  This,  the  longest  lateral, 
branched  from  the  main  root  3  cm.  from  the  surface,  and  lay  21,  39,  31, 
and  28  cm.  from  the  surface  of  the  ground  at  various  distances  from  its 
base.  The  position  of  this  root  with  relation  to  the  surface  is  fairly  illus- 
trative of  all  of  the  rest.     The  mutual  relations  of  the  principal  roots, 


6o  ROOT   HABITS    OF   DESERT   PLANTS. 

together  with  vertical  extension  of  two  plants  from  West  Wash,  are  shown 
in  fig.  6.  The  general  character  of  the  bases  and  the  number  of  smaller  laterals 
are  adequately  shown  in  plate  i6.  Besides  the  larger  branches  of  the  main 
laterals,  which,  as  fig.  6  shows,  were  relatively  few,  there  were  groups  of 
filamentous  roots  occurring  along  their  course  as  in  Franseria  and  other 
plants.  Although  not  specially  studied  in  Covillea,  these  rootlets  were  of 
limited  growth,  ser^dng  the  plant  for  a  short  period  only,  as  in  the  species 
mentioned. 

The  soil  of  the  flood-plain  by  West  Wash,  where  the  other  specimen  of 
Covillea  studied  was  growing,  was  of  adobe  and  sand  to  a  depth  exceeding 
2.1  meters.  The  perennials  of  the  habitat,  both  as  regards  kinds  and  num- 
bers, were  more  numerous  than  on  the  bajada.  Near  the  specimen  of 
Covillea  from  the  flood-plain,  whose  roots  were  examined,  were  Acacia 
greggii,  Lyciuni  andersonii,  Parkinsonia  torreyana,  Prosopis  velutina,  and 
other  forms,  mainly  shrubs. 

The  Covillea  studied  by  West  Wash  was  1.65  meters  high,  or  somewhat 
larger  than  the  bajada  specimen  examined,  and,  from  the  size  and  number 
of  the  leaves  and  the  character  of  the  branching,  it  gave  evidence  of  having 
a  better  water  supply  than  the  bajada-grown  plants.  An  examination  of 
the  root-system  of  this  plant  showed  that  in  certain  ways  it  was  very  dif- 
ferent from  that  of  the  plant  from  the  bajada.  The  points  of  difference 
appear  well  in  comparing  the  vertical  extension  of  the  roots  of  the  two 
plants,  as  shown  in  the  figures.  The  tap  root  was  traced  to  its  end  at  a 
depth  of  30  cm.,  although  in  another  specimen  growing  near  by  the  tap  root 
went  down  1.7  meters,  indicating  that  in  the  specimen  under  consideration 
some  calamity  had  brought  about  the  untimely  end  of  the  main  root.  The 
laterals  were,  in  part,  given  ofif  from  the  bases  of  the  shoots  and,  after 
leaving  the  main  plant  axis,  had  little  uniformity  in  position.  Some  lay 
20  to  45  cm.  from  the  surface,  others  were  still  deeper.  Such  roots  took 
a  more  or  less  horizontal  position.  From  these  laterals  branches  were 
given  off,  one  of  which  went  straight  downward  as  far  as  2.1  meters. 

Although  the  laterals  of  the  first  (?)  order  ran  fairly  horizontally  as  a 
whole,  they  were  not  as  long  as  the  corresponding  roots  of  the  bajada- 
grown  plant;  the  radius  included  within  the  root-area  did  not  exceed  2.2 
meters,  as  opposed  to  nearly  twice  this  figure  in  the  other  form  studied.  In 
addition  to  the  larger  laterals  just  described,  many  smaller  ones  arose  from 
the  bases  of  the  shoots  or  from  the  bases  of  the  larger  laterals.  As  plate  16 
shows,  there  were  in  the  plant  from  the  flood -plain  man}^  more  small  adven- 
titious roots  than  in  the  plant  from  the  bajada.  The  laterals  bore  relatively 
few  branches. 

The  root-system  of  the  specimen  of  Covillea  from  the  flood-plain  may 
be  characterized,  therefore,  as  having  a  deeply  penetrating  main  root  and 
laterals  which  run  in  a  fairly  horizontal  direction,  although  they  may  lie 
as  deep  as  53  cm.  and  may  give  off  branches  that  go  straight  downward. 


ROOT-SYSTEMS    OF   PERENNIALS.  6 1 

The  number  of  small  laterals  at  the  bases  of  the  plant  is  especially  large. 
The  roots  are  mainly  either  horizontal  or  vertical,  without  being  at  any 
intermediate  angle,  a  character  so  strong  in  the  root-system  of  Prosopis, 
to  be  described  later,  and  of  so  wide  occurrence  among  the  larger  plants, 
where  the  soil  conditions  are  favorable,  that  it  forms  one  of  the  most  striking 
of  the  general  root  characters  of  desert  plants. 

The  lateral  extension  of  the  roots  of  the  Covillea  from  the  flood-plain  is 
considerably  less  than  that  of  the  plant  from  the  bajada.  Those  of  the 
latter  reach  out  over  4  meters,  while  the  roots  of  the  former,  as  mentioned 
above,  extend  hardly  half  as  far.  The  root-area,  or  the  area  included 
within  the  reach  of  the  roots  of  the  bajada-grown  Covillea,  was  about  50 
square  meters,  while  that  of  the  plant  from  the  flood-plain  was  only  one- 
fourth  as  much.  These  differences  make  it  imperative  that  fewer  plants 
occupy  a  given  space  on  the  bajada  than  on  the  flood-plain.  It  is  of  interest, 
however,  to  note  that  the  mass  of  earth  compassed  by  the  root-system  of 
the  two  specimens  of  Covillea  was  approximately  equal. 

With  the  characteristic  differences  in  root  development  and  root  posi- 
tion of  the  bajada  and  flood-plain  plants  are  associated  dift'erences  in  the 
relations  of  these  plants  to  their  neighbors.  In  excavating  the  roots  of  the 
specimen  from  the  flood-plain  no  roots  of  other  woody  plants  were  encoun- 
tered within  the  root-area  of  the  plant  studied.  But  the  condition  of  the 
bajada-grown  Covillea  was  quite  otherwise.  No  fewer  than  60  roots  of 
neighboring  Covilleas  were  encountered  at  the  same  level,  and  frequently 
in  physical  contact  with  the  roots  of  the  plant  studied,  and  very  many 
more  were  met  between  the  levels  of  these  roots  and  the  surface  of  the 
ground.  Therefore  the  competition  between  neighboring  Covilleas  on  the 
bajada,  for  soil  water,  is  presumably  keen,  while,  on  the  other  hand,  com- 
petition between  neighboring  Covilleas  on  the  flood-plain,  for  soil  water,  is 
at  best  indirect,  and  may  be  so  slight  as  to  be  neghgible. 

FOUOUIERIA  SpLENDENS. 

Fouquieria  occurs  most  abundantly  on  the  northern  slope  of  Tumamoc 
Hill,  on  its  lower  drainage  slope,  or  the  upper  portion  of  the  bajada,  and 
on  the  bajada  near  West  Wash.  It  was  not  found  on  the  flood-plain  of 
West  Wash  or  of  the  Santa  Cruz,  that  is,  in  other  words,  the  species  in  the 
vicinity  of  the  Desert  Laboratory  does  not  grow  where  the  soil  has  consider- 
able depth,  so  that  the  "normal"  behavior  of  its  roots  is  not  known. 

The  soil  conditions  where  the  specimens  of  Fouquieria  specially  studied 
were  growing,  the  bajada  to  the  north  of  Tumamoc  Hill,  are  nearly  the 
same  as  those  of  the  Covillea  habitat  above  described,  except  that  the  super- 
ficial adobe  clay  is  somewhat  thicker  and  there  may  be  a  greater  admixture 
of  small  stones.  The  superficial  soil  is  underlaid  by  rotten  caliche  and  the 
latter  by  hard  caliche,  as  in  the  habitat  referred  to. 


62 


ROOT   HABITS    OF   DESERT   PLANTS. 


Fig.  7. — Root  and  shoot  habit  of  Fouquicria  splcndens. 


ROOT-SVSTIJMS    OF    PERENNIALS. 


63 


The  other  woody  plants  growing  near  Fouqiiieria  were  mainly  Covillea 
tiidentafa,  and  an  occasional  specimen  of  Parkinsonia  micro phylla. 

Two  specimens  of  Fouqiiieria  were  specially  studied,  of  which  one  was 
young  and  the  other  mature.  The  younger  plant  had  a  shoot  24  cm.  high, 
which  had  a  central  main  axis  and  two  branches  3  cm.  and  15  cm.  long. 
The  habit  of  the  plant  is  well  shown  in  fig.  7. 

The  roots  of  Fouquieria  splendens  are  coarse  and  brittle,  and  bear  rela- 
tively few  branches.  The  tap  root  was  large  and  penetrated  the  ground 
S  cm.,  where  it  forked,  one  branch  extending  horizontally  13  cm.  and  the 
other  going  down  18  cm.  before  branching.  Each  of  the  main  forks  of  the 
main  root  ])ranched  onlv  two  or  three  times. 


Fig.  S.—Honzontal  extension  of  the  root-system  of  Fouquieria  splendens  from  the  bajada  at 
the  north  base  of  Tumamoc  Hill.  Dots  represent  position  of  Covillea  liideniat a, which 
occurred  withm  the  root-area  of  Fouquieria,  and  dotted  lines  such  roots  of  Covillea  as 
were  found  on  the  same  horizon  as  the  Fouquieria  roots. 

Above  the  forking  of  the  tap  root,  about  3  to  4  cm.  from  the  surface  of 
the  ground,  two  laterals  arose,  which,  as  far  as  traced,  ran  in  a  nearly  hori- 
zontal direction.  One  of  these  bore  3  branches  and  was  over  22  cm.  long. 
At  the  base  the  laterals  were  less  than  i  cm.  in  diameter,  and,  where  broken 
off,  they  were  slightly  more  than  i  mm.  in  diameter.  Thus  the  roots  of 
Fouquieria  are  so  heavy  that  they  may  perhaps  be  termed  fleshy. 


64  ROOT   HABITS    OF    DESERT   PLANTS. 

'  A  character  of  the  smaller  roots  of  the  plant  was  the  bearing  of  groups  of 
filamentous,  adventitious  rootlets  about  i  cm.  in  length.  At  the  time  the 
study  was  made,  January,  these  rootlets  were  dead. 

The  only  roots  of  neighboring  plants  which  intruded  on  the  root-area  of 
Fouquieria  were  two  of  Parkinsonia  micro phylla,  which  lay  3  and  15  cm. 
from  the  surface  of  the  ground. 

The  larger  specimen  of  Fouquieria  examined  was  1.75  meters  high  and 
bore  numerous  long  branches.  In  February,  when  the  study  was  made,  the 
plant  was  without  leaves. 

The  following  were  the  main  characters  of  the  root-system  of  the  larger 
plant.  A  tap  root  penetrated  the  ground  until  it  met  a  mass  of  caliche 
which  lay  immediately  below  the  base  of  the  plant.  The  depth  attained  by 
the  root  was  about  15  cm.  Five  laterals  arose  immediately  below  the  sur- 
face of  the  ground ;  they  were  5  cm.,  more  or  less,  in  diameter,  at  the  base. 
It  is  a  characteristic  of  Fouquieria  that  the  roots,  although  relatively  heavy 
at  the  base,  taper  rapidly;  for  example,  one  5  cm.  in  diameter  at  its  base 
had  a  diameter  of  i.i  cm.  about  50  cm.  away.  After  the  diameter  of  i.o  cm. 
is  reached,  the  root  maintains  this  very  closely  for  a  considerable  distance. 

The  depth  usually  attained  by  the  laterals  may  be  illustrated  by  a  few 
examples.  Taking  a  typical  root,  we  find  that  while  it  leaves  the  tap  root 
immediately  beneath  the  surface  of  the  ground,  at  a  distance  of  40  cm.  it 
is  25  cm.  deep ;  at  50  cm.  it  is  16  cm.  deep ;  at  a  distance  of  i  meter  it  is  23 
cm.  beneath  the  surface;  and  at  a  point  1.75  meters  from  the  main  axis, 
where  it  is  2  mm.  in  diameter,  the  root  is  21  cm.  deep.  A  branch  of  this 
root  was  traced  downward  to  a  depth  of  37.5  cm.,  but  at  that  point  it  was 
no  longer  living. 

Groups  of  filamentous  roots  were  not  seen  in  the  larger  specimen.  There 
were  several  specimens  of  Covillea  in  the  area  included  within  the  scope  of 
the  roots  of  Fouquieria.  One  was  about  25  cm.  distant  and  its  roots  occu- 
pied much  the  same  area  as  those  of  the  Fouquieria,  but  they  were  deeper 
and,  although  the  roots  of  Fouquieria  were  exposed  with  great  care,  only 
four  instances  were  noted  where  roots  of  the  neighboring  Covilleas  were 
found  in  the  same  horizon  as  those  of  the  plant  examined.  Competition 
between  the  two  species  of  shrubs,  therefore,  is  probably  not  very  active. 

Franseria  Deltoidea. 

Although  perhaps  as  widely  distributed  locally  as  Covillea,  the  character- 
istic habitat  of  Franseria  deltoidea  on  the  domain  of  the  Desert  Laboratory 
is  the  bajada,  just  to  the  west  of  West  Wash.  In  this  habitat  the  popula- 
tion of  Franseria  for  any  given  area  is  greater  than  that  of  any  other  shrub 
in  its  characteristic  habitat  in  this  vicinity.  In  fact,  the  species  at  the 
place  in  question  fairly  completely  conceals  the  ground. 

In  the  neighborhood  of  the  Franseria  examined,  the  following  woody 
plants  also  occurred:  Covillea  tridentata,   Krameria  canescens,   Krameria 


i     \   "N^ 


A.  Covillea  tridentata  from  bajada  at  north  base  of  Tumamoc  Hill.  Horizon- 
tal and  vertical  extensions  of  roots  of  this  plant  shown  in  fig.  ti. 

B.  Covillea  from  flood-plain  near  West  Wash,  showing  the  large  number  of 
slender  adventitious  roots  springing  from  upper  part  of  roots  and  long  tap-root,  of 
which  only  a  portion  appears. 

C.  Fouquieria  splendens  from  bajada  near  where  Covillea  shown  in  A  was 
growing.     Horizontal  extension  of  roots  of  this  plant  shown  in  fig.  8. 


a.  Franseria  deltoidea  from  flood-plain,  February,  1909,  showing 
part  of  tap-root.  b.  Lateral  of  the  first  order  bearing  filamentous  roots 
in  groups,  which,  at  time  photograph  was  made,  were  no  longer 
functional.  c.  Krameria  canescens  from  flood-plain  near  West 
Wash,  February,  1909,  showing  essential  superficial  placing  of  roots, 
d.   Lycium  andersonii  from  near  West  Wash. 


ROOT-SYSTEMS   OF    PERENNIAI.S. 


65 


parvifolia,  Opuntia  discata,  Opuntia  versicolor,  other  species  of  Opuntia,  and 
Parkinsonia  microphylla. 

The  soil  of  the  habitat  is  much  hke  that  of  the  portion  of  the  bajada  at 
the  northern  base  of  Tumamoc  Hill,  where  Covillea  was  studied,  adobe  clay 
constituting  the  upper  soil  to  about  20  cm.  depth,  under  which  is  rotten 
caliche,  the  latter  resting  on  hard  caliche.  The  situation  is  probably  the 
most  severely  arid  of  any  habitat  studied. 


Fig.  9. — Vertical  extension  of  the  root-system  of  Franseria  deltoidca  from 
different  habitats,  a,  root-system  of  a  bajada-grown  plant.  Upper 
surface  of  caliche  hardpan  indicated  by  dotted  line.  6,  root-system  of 
a  plant  from  the  flood-plain  by  West  Wash  to  show  depth  to  which 
tap  root  penetrated,  1.8  m. 

Two  specimens  of  Franseria  from  the  bajada  habitat  and  one  from  the 
flood-plain  of  West  Wash  were  examined.  Therefore,  as  regards  soil  con- 
ditions and  water  relations,  the  two  habitats  present  the  greatest  contrast 
to  be  found  on  the  domain. 


66  Rcxrr  habits  of  desert  plants. 

The  shoot  of  the  larger  specimen  of  Franscria  from  the  bajada  was  50 
cm.  high  and  was  composed  of  numerous  slender  branches.  The  species 
is  well  covered  with  leaves,  so  that,  compared  with  most  of  the  local  shrubs, 
it  has  a  large  transpiration  surface.  The  leading  characters  of  the  shoot 
are  shown,  but  rather  imperfectly,  in  plate  17. 

The  root-system  consisted  of  a  tap  root  bearing  several  laterals  and  of 
numerous  adventitious  roots  originating  from  the  bases  of  the  shoot.  The 
tap  root  went  straight  down  29  cm.,  where  it  forked,  one  root  penetrating  the 
rotten  caliche  to  a  depth  of  38  cm.  from  the  surface  of  the  ground.  About 
12  cm.  from  the  surface,  the  main  root  gave  off  several  laterals  which 
reached  as  far  as  1.6  meters  from  the  main  axis  and  lay  12  to  30  cm.  deep. 
Thus  the  roots  are  confined  to  soil  above  the  hard  caliche.  The  roots  are 
dark  brown,  very  brittle,  and  are  ridged  longitudinally  with  cork.  They 
are  always  slender;  the  largest  root,  the  tap  root,  2  cm.  from  the  crown, 
was  only  8  mm.  in  diameter. 

The  younger  specimen  of  Franseria  studied  was  3  meters  distant.  Its 
shoot  was  14  cm.  high  and  composed  of  four  separate  branches.  Upon  dig- 
ging, it  was  learned  that  the  surface  soil  was  relatively  deep,  and  that  over 
a  small  ar£a  the  hard  caliche  had  given  place  to  adobe,  a  condition  similar 
to  that  shown  in  plate  i. 

A  main  root  of  the  young  plant  was  traced  down  i  meter,  and  evidently 
penetrated  farther.  The  main  root  gave  off  one  lateral  3  cm.  long,  another 
less  than  10  cm.  long,  and  a  third  which,  shortly  after  leaving  the  main 
root,  penetrated  to  a  depth  of  60  cm.  The  vertical  extension  of  the  root- 
system  is  shown  in  fig.  9.  No  roots  of  neighboring  plants  were  seen  in  the 
root-areas  of  this  plant. 

For  comparative  purposes,  the  roots  of  a  Franscria  growing  on  the  flood- 
plain  not  far  to  the  east  of  the  proper  habitat  of  the  species  were  also 
examined.  The  soil  is  a  sandy  loam,  sand  and  adobe,  of  a  depth  exceeding 
2  meters.  No  excavations  have  been  made  below  this  level.  The  place  is 
occasionally  flooded  and  the  water-table  is  variable  as  to  its  depth,  but 
probably  the  ground  at  a  depth  reached  by  roots  is  moist  all  of  the  year. 

The  leading  woody  plants  of  the  flood-plain  in  the  immediate  neighbor- 
hood of  the  specimen  of  Franseria  studied  were  Acacia  greggii,  Covillea 
tridentata,  Ephedra  trifurca,  Lyciion  andersonii,  Parkinsonia  torreyana,  and 
Prosopis  velutina.  The  flood-plain  is  the  characteristic  habitat  of  all  of 
these  except  Covillea  and  Franseria. 

The  leading  points  of  interest  in  the  root-system  of  Franseria  from  the 
flood-plain  were  as  follows:  A  tap  root  which  went  straight  dow^n  to  a  depth 
of  1.8  meters  w^as  only  5  mm.  in  diameter  at  its  crown.  For  the  most  part 
the  laterals  arose  within  15  cm.  of  the  surface,  and  one  of  these  extended 
1.4  meters  in  a  fairly  horizontal  direction  (see  fig.  9) ;  another  took  a  doAvn- 
ward  course  at  an  angle  approximating  45°  until  a  depth  of  45  cm.  was 
reached,  after  which  it  was  approximately  horizontal;  a  third  root,  after 


ROOT-SYSTEMS   OF    PERENNIALS.  67 

reaching  out  about  20  cm.  from  its  place  of  origin,  turned  directly  down- 
ward to  a  depth  of  78  cm. 

Besides  roots  arising  on  the  main  root,  and  of  primary  origin,  there  were 
over  50  slender  adventitious  roots,  about  3  mm.  in  diameter,  which  arose 
from  the  shoot  bases  and  from  the  crown  of  the  main  root.  For  the  most 
part,  these  roots  took  a  horizontal  direction,  but  one  of  them  ran  downward 
and  was  traced  to  its  end  at  a  depth  of  1.3  meters.  Such  laterals  as  lay 
about  15  cm.  from  the  surface  of  the  ground  bore  groups  of  filamentous 
roots  (plate  17),  which  were  dead  when  the  study  was  made.  The  rootlets 
were  2  cm.,  more  or  less,  in  length  and  were  six  or  so  in  a  group ;  the  groups 
were  from  5  mm.  to  2  cm.  apart. 

Filamentous  roots  were  seen  also  in  the  bajada-grown  plants,  and  in 
others  from  Tumamoc  Hill.  In  January,  19 10,  after  the  soil  had  been 
moistened  some  days  by  rains,  freshly  formed  rootlets  were  seen  on  many 
roots  of  Fransena.  These  roots,  therefore,  are  formed  in  winter,  and 
probably  in  summer  as  well,  and  are  very  short-lived.  No  study  has  yet 
been  made  of  the  conditions  under  which  the  rootlets  are  developed,  their 
period  of  activity,  or  whether,  if  conditions  remained  constantlv  favorable, 
they  would  be  active  indefinitely. 

Krameria  Canescens. 

One  of  the  most  generally  distributed  species  in  the  vicinity  of  the 
Desert  Laboratory  is  Krameria  canescens,  occurring  as  it  does  on  each  of  the 
physiograpliic  areas  treated  in  the  present  paper.  It  attains  its  largest  size 
where  the  water  relations  are  most  favorable,  as  on  the  flood-plain  of  the 
wash,  on  the  bajada  near  it,  and  on  Tumamoc  Hill.  As  has  been  shown 
elsewhere*  Krameria  is  an  habitual  parasite  and  this  is  probably  the  lead- 
ing cause  for  the  wide  distribution  noted. 

The  first  specimen  of  Krameria,  of  which  the  root-system  was  examined, 
was  an  isolated  plant  on  the  flood-plain  of  West  Wash.  The  soil  condi- 
tions of  the  place  have  already  been  noted ;  in  brief  they  were :  a  sandy  clay 
over  2  meters  deep  and  relatively  favorable  water  relations,  as  evidenced 
by  the  occurrence  of  the  most  hygrophilous  of  the  native  species. 

The  specimen  studied  had  a  much-branched  shoot,  about  60  cm.  high, 
which  was  leafless  in  February.  The  general  character  of  the  shoot  is  shown 
in  plate  1 7.  The  root-system  consisted  of  a  short  tap  root,  no  longer  living, 
and  six,  or  more,  laterals  which  extended  as  far  as  2  meters  from  the  main 
axis  (fig.  10) .  Besides  the  main  laterals  there  w^ere  about  an  equal  number 
of  more  slender  ones,  less  than  2  mm.  in  diameter  and  about  20  cm.  long, 
which  ran  in  a  horizontal  direction.  The  longer  roots,  from  4  mm.  to  i  cm. 
in  diameter  at  the  base,  lay  from  13  to  18  cm.  beneath  the  surface.  In 
addition  to  these  roots,  several  were  dead  and  decaying.     A  very  striking 

^Conditions  of  Parasitism  in  Plants,  by  D.  T.lviacDougal  and  W.  A.  Cannon  Publi- 
cation  ^o.  129,  Carnegie  Institution  of  Washington,  1910. 


68 


ROOT   HABITS    OF    DESERT    PLANTS. 


Fig.  10. — Root-system  of  Krameria  canescens. 
The  large  laterals  d,  d,  d  were  dead. 


feature  of  the  root-system  of  the  plant  was  the  almost  total  absence  of 
branches,  nor  were  the  filamentous  roots  which  occur  in  Franseria,  Encelia, 
and  in  other  perennials,  obser\^ed  in  Krameria. 

Only  two  foreign  roots 
(shown  as  dotted  lines,  fig. 
lo)  of  an  unknown  species 
were  encountered  during 
the  excavation  of  roots  of 
Kravieria. 

The  presence  of  so  many 
dead  roots  made  it  advis- 
able to  examine  another 
specimen  of  equal  age  in 
order  that  a  perfectly  "nor- 
mal" root-system  should 
be  described.  Accordingly, 
another  plant,  also  growing 
on  the  flood-plain,  was 
selected  for  study.  The 
results  of  the  second  excava- 
tion substantiated  those  of 
the  first,  namely,  that  the  tap  root  had  reached  a  rather  large  size  before 
dying,  that  there  were  many  dead  laterals,  and  that  those  yet  living  were 
near  the  surface,  although  the  soil  at  the  place  was  of  considerable  depth. 
In  examining  another  plant,  under  circumstances  which  allowed  the  obser- 
vation of  the  relation  of  the  roots  of  the  species  to  those  of  its  neighbors, 
the  fact  of  the  parasitic  habit  of  the  plant  was  revealed. 

After  the  discovery  of  parasitism  in  Krameria  a  very  large  number  of 
specimens  growing  under  widely  different  conditions  were  examined  and 
root  attachment  to  the  following  plants  was  demonstrated:  Acacia  con- 
stricta,  Covillea  tridentata,  Encelia  farinosa,  Ephedra  trifurca,  Franseria 
deltoidea,  Lyciwn  andersonii,  Menodora  scabra,  Opuntia  sp.,  Parkins onia 
microphylla,  Prosopis  velutina,  and  Zizyphus  parryi.  Subsequently  para- 
sitism of  the  plant  on  Parkinsonia  aculeata,  a  species  indigenous  within  75 
miles  of  Tucson,  was  induced  in  cultures. 

The  attachment  to  the  host  is  usually  by  means  of  small  roots  and  only 
rarely  does  one  find  the  association  of  old  roots  of  host  and  parasite.  Thus 
the  connection  is  probably  made  during  the  growing  seasons,  and  persists 
if  the  conditions  continue  favorable  during  the  subsequent  dry  periods. 

The  hosts  already  found  are  only  trees  and  shrubs,  and  it  is  not  at  present 
known  whether  the  parasite  is  strictly  limited  to  such,  or  whether  it  may 
not  derive  temporary  sustenance  from  annuals,  some  of  which  are  very 
resistant  to  drought  and  endure  for  considerable  time  after  the  rainy  seasons 
have  closed. 


^^Hp  ^           .     --''\f^^^^' 

^K'            '^"■^'  '^■■ 

PPHH 

,,-*=?sii;,,  , 

"^^ 

Bird's-eye  view  of  superficial  roots  of  Opuntia  arbuscula,  from  bajufla, 
a  mile  east  of  the  Santa  Cruz,  showing  their  fleshy  character. 


ROOT-SYSTEMS   OF   PERENNIALS.  69 

The  root-system  of  the  seedhng  has  a  pronounced  tap  root  with  relatively 
few  and  short  laterals  which  do  not  bear  haustoria  if  the  plant  is  growing 
alone,  but  develop  them  if  the  roots  of  another  form,  such  as  Parkinsonia 
acnleata,  are  in  close  relation  with  its  own.  The  root-system  of  the  seed- 
ling, however,  differs  from  that  of  autotrophic  plants  in  the  fact  that  it  does 
not  form  root-hairs.  As  the  plant  develops,  the  tap  root  continues  to  be  a 
leading  feature,  since  in  the  old  plants  remains  of  the  tap  root  have  always 
been  encountered.  When  or  why  this  organ  dies  is  not  known.  In  the 
mature  plant  the  superficial  laterals  become  the  most  prominent  feature  of 
the  root-system  and  include  a  large  area  within  their  compass. 

In  associating  character  of  root-system  with  habit  in  any  species,  it  has 
repeatedly  been  shown  above  that  an  autotrophic  plant  without  water 
reserve  is  always  provided  with  a  deeply  placed  root-system,  or  as  deeply 
as  the  soil  permits,  but  that  a  plant  with  water-storage  facility  has  a  root- 
system  which  lies  near  the  surface  of  the  soil.  In  Krameria  the  roots  are 
shallow,  although  the  form  is  not  a  water-storing  species — suggestive  of 
unusual  life-habit  of  the  species. 

Opuntia  Arbuscula. 

Among  the  species  of  cacti  limited  to  or  characteristic  of  the  bajada  are 
Opuntia  arbuscula  and  0.  fulgida,  though  the  latter  only  occurs  on  the 
domain  of  the  Desert  I^aboratory  The  habit  of  the  two  arborescent  cylin- 
dro-opuntias  is  unlike.  The  species  arbuscula  occurs  in  groups  of  few 
individuals  usually,  of  common  descent,  as  will  be  shown,  while /w/^ifia  at  its 
best  forms  fairly  dense  growths,  to  the  exclusion  of  other  woody  plants. 

The  specimen  of  Opuntia  arbuscula  chiefly  studied  was  one  of  a  small 
group  growing  on  the  bajada  about  a  mile  east  of  the  vSantaCruz  river,  April, 
1907.  The  upper  soil  is  of  adobe  clay  to  a  thickness  approximating  20  cm. 
with  a  layer  of  rotten  caliche  beneath,  of  variable  thickness,  which  rests  on 
the  hardpan. 

In  the  vicinity  of  the  cactus  were  Acacia  constricta,  Bigelowia  hartwegii, 
Covillca  tridentata,  Krameria  canescens,  Opuntia  fulgida,  and  0.  spinosior. 
Of  these  species,  3  plants  of  Covillea  and  one  of  Bigeloivia  occurred  within 
the  root-area  of  the  cactus.  The  Covilleas  were  20  cm.  west,  70  cm.  south, 
and  2  meters  northeast,  all  mature  shrubs  of  large  size. 

The  Opuntia  whose  roots  were  examined  was  a  much  branched  and  old 
plant  about  a  meter  high,  apparently  in  perfect  health  and  in  every  way  a 
normal  specimen.  Its  root-system  was  composed  of  an  anchoring  portion 
and  a  widely  reaching  horizontal  portion  (fig.  11). 

The  anchoring  roots  were  not  specially  studied.  The  horizontal  system 
consisted  of  4  main  members  arising  from  the  crown  of  the  tap  root  just 
below  the  surface  of  the  ground.  They  radiated  from  the  main  axis  in  such 
fashion  that  the  ground  around  the  base  of  the  plant  was  fairly  equally 
divided  between  them.     These  main  laterals,  as  distinguished  from  the  cor- 


70  ROOT   HABITS    OF    DESKRT    PLANTS. 

responding  roots  of  all  other  opuntias  seen,  branched  repeatedly  through- 
out their  course,  in  places  forming  a  network  closely  covering  the  ground. 
The  laterals  extended  for  about  3  meters  and  lay  so  near  the  surface  of 
the  ground  that  the  tips  could  be  lifted  out  with  a  walking-stick,  and  they 
could  be  torn  out  of  the  ground  their  entire  length.  More  exactly  stated, 
the  roots  lie  from  2  cm.,  and  even  less,  to  8  cm.  beneath  the  surface. 


Fig.  11. — Horizontal  extension  of  root-system  of  Opuntia  arhuscula.     Black  dots  indicate  position 
of  Covillea  tridentata  which  had  invaded  the  root-area  of  the  cactus. 

As  compared  with  the  roots  of  other  opuntias,  those  of  arbiiscula  are 
large  (plate  18).  This  will  be  appreciated  from  the  following  measure- 
ments. A  meter  from  its  base  one  of  the  larger  roots  had  a  diameter  of 
2.5  cm.,  2  meters  distant  it  was  2  cm.,  3  meters  distant  it  was  1.5  cm.  The 
tips  of  these  roots,  excluding  the  most  recent  growth,  were  not  less  than 
I  cm.  in  diameter.  From  the  size  and  tuber-like  appearance  of  the  roots  it 
was  concluded  that  they  function  as  water-storing  organs.  This  conclusion 
was  strengthened  by  the  observation  that  inside  of  48  hours  after  the  roots 
of  this  plant  were  removed  from  the  soil  they  were  rapidly  losing  their 
plump  aspect  and  had  begun  to  shrivel  strikingly.  A  noteworthy  peculi- 
arity of  the  larger  roots  was  their  frequent  abrupt  ending  through  the  death 
of  the  more  distal  portion. 

The  fleshy  roots  of  arhuscula,  in  addition  to  functioning  as  water-storing 
organs,  a  rare  root-character  among  mature  opuntias,  further  serve  as  prop- 
agating organs,  and  to  this  function  the  occurrence  of  the  species  in  masses 
is  due.  The  procedure,  in  brief,  is  this.  One  or  more  shoots  may  arise 
from  a  single  lateral  while  the  latter  retains  its  connection  with  the  parent. 


ROOT-SYSTEMS    OF    PEREXXIAI.S.  71 

The  shoots  put  down  roots,  the  connection  with  the  parent  plant  is  prob- 
ably early  cut  off  (when  was  not  learned),  and  the  daughter  plant  is  inde- 
pendent. The  result  is  that  arbuscula  frequently  forms  clumps  several 
meters  in  diameter,  to  the  total  exclusion  of  other  species. 

From  the  previous  description  of  the  root-system  of  Covillca  it  is  perhaps 
hardly  necessary  to  point  out  that  the  roots  of  this  species  growing  within 
the  root-area  of  the  cactus  are  much  deeper  than  the  roots  of  the  latter 
plant,  and  hence  are  not  in  active  competition  with  the  cactus  roots  for 
their  water  supply. 

Opuntia  Fulgida. 

On  the  domain  of  the  Desert  Laboratory,  Opuntia  fulgida  occurs  only 
on  the  bajada  near  West  Wash,  where  in  places  it  forms  fairly  dense  growths 
constituting  the  most  prevalent  species.  This  is  an  unusual  habit  for  any 
cactus  in  this  vicinity,  since  usually  the  cacti  are  either  scattered  in  an 
apparently  chance  fashion,  unless  controlled  by  such  external  factors  as 
temperature  or  other  physical  environmental  conditions,  or  at  best  but 
few  indi^•iduals  of  a  species  are  congregated. 

The  soil  in  the  habitat  of  fulgida  is  a  sandy  clay  with  an  admixture  of 
rocks  of  various  sizes  which  appeared  to  be  in  stages  of  active  disintegra- 
tion. No  caliche  was  found  to  a  depth  of  60  cm.  The  habitat,  therefore, 
presents  certain  striking  differences  from  that  of  Franseria,  not  far  distant. 

In  the  immediate  vicinity  of  the  Opuntia  studied  were  Acacia  constricta, 
Echinoccreus  fcndlcri,  Covillea  tridcntata,  Opuntia  fulgida,  Opuntia  versicolor, 
Parkinsonia  micro  phylla,  and  Riddel  I  ia  coo  peri,  and  of  these  plants  only  one 
example  of  Acacia  and  two  of  Riddel  Ha  occurred  within  the  root-area  of  the 
cactus.  vSince,  as  will  appear  directly,  the  roots  of  fulgida  extended  as 
far  as  3  meters  from  the  main  plant  axis,  the  perennial  population  of  the 
habitat  is  relatively  light. 

The  specimen  selected  for  study  was  growing  somewhat  apart  from  the 
typical  fulgida  habitat,  but  the  plant  appeared  to  represent  thoroughly  the 
typical  and  mature  species.  The  root-system,  like  that  of  Opuntia  versi- 
color, has  a  somewhat  deeply  penetrating  portion,  the  anchoring  roots,  and 
a  widely  reaching  horizontal  portion  not  far  from  the  surface  of  the  ground. 
The  horizontal  and  vertical  extensions  of  the  root-svstem  are  shown  in 
fig-  12. 

The  anchoring  system  consisted  of  about  six  stout  roots  which  ran  down 
at  an  acute  angle  to  a  depth  of  35  cm.  Of  these  the  largest  was  2.5  cm.  at 
the  crown ;  it  tapered  rapidly  and  gave  off  several  branches  which  ran  out- 
ward for  a  distance  of  5  to  10  cm.  The  anchoring  roots  did  not  reach  hard- 
pan  or  bed-rock,  or  other  material  which  might  prevent  deeper  penetration. 
The  superficial  root-system  consisted  of  6  main  laterals,  of  which  4  ran  3 
meters,  more  or  less,  in  an  easterly  direction  (down-hill),  and  the  others 
took  a  fairly  well-defined  up-hill  direction.  The  roots  branched  but  little; 
for  example,  one  3  meters  long  gave  off  no  important  branches,  and  a  some- 


72 


ROOT  HABITS   OF   DESERT  PLANTS. 


what  larger  one  branched  only  twice.  The  roots  of  no  cactus  studied,  and 
in  fact  no  perennial  plant  except  the  parasite  Kramcria,  branched  less  than 
this  one. 

The  roots  were  slender,  as  the  following  representative  measurements 
will  indicate.  One  of  the  superficial  roots  which  was  3  meters  in  length 
was  6  mm.  in  diameter  35  cm.  from  the  base;  another  long  lateral,  1.2 
meters  from  its  base,  was  3  mm.  in  diameter;  and  a  third  root,  1.5  meters 
from  the  point  of  origin  on  the  main  root,  was  4  mm.  in  diameter.     This 


Fir,.  12. — Root-system  of  Opiinlia  fulgidn.  In  the  sketch  of  the  horizontal  extension  of  O.  fulgida, 
A,  two  specimens  of  Riddellia  cooperi  (a)  and  one  Acacia  constncta  (b)  are  shown.  The 
vertical  extension  of  the  roots  of  O.  julgida  is  given  in  b,  and  of  Acacia  conslrKta  in  c. 

system  was  also  shallow,  the  main  lateral  running  southwest,  at  a  point  35 
cm.  from  the  central  axis,  was  4  cm.  deep,  and  at  a  point  1.2  meters  from 
the  main  plant  axis  it  was  6  cm.  beneath  the  surface  of  the  ground.  The 
other  laterals  also  were  found  to  lie  5  to  6  cm.  below  the  surface,  although, 
as  was  pointed  out  above,  the  soil  at  the  place  was  relatively  deep  and 
would  have  offered  no  obstacles  to  deeper  root  penetration. 

The  roots  of  the  3  specimens  of  Riddellia  cooperi,  growing  within  the 
root-area  of  the  cactus,  were  also  examined.  They  each  sent  a  tap  root 
straight  down,  and  in  each  root  a  few  laterals  were  given  off  which  ran  in 
a  more  or  less  horizontal  direction  as  far  as  10  cm.  How  deep  the  tap 
root  of  Riddellia  penetrated  was  not  learned.  At  no  place,  however,  did 
the  root-systems  of  Riddellia  appear  to  occupy  the  same  horizon  as  the 


n^K^m^mts 


A.  Riddellia  cooperi,  from  sandy  loam  east  of  West  Wash,  showing  the  large  num- 
ber of  slender  laterals  arising  from  crown  of  tap-root  and  the  branching  feature  of 
the  bases  of  the  main  laterals.  B.  Riddellia  from  red  claj^  soil  near  West  Wash,  to 
illustrate  lack  of  slender  roots  at  crown  of  tap-root. 

C.  Dasylirion  texanum,  from  bajada  near  Rincon  mountains,  with  root-system 
partly  exposed. 

D.  Shoot  and  root  habit  of  Yucca  sp.,  from  bajada  near  Rincons.  Plant  supported 
by  two  props,  one  at  side  and  one  below,  which  should  not  be  mistaken  for  roots. 

E.  Root  character  of  Yucca  radiosa  from  bajada  near  the  Rincons. 


ROOT-SYSTEMS    OF    PERENNIALS.  73 

roots  of  the  cactus,  so  that  active  competition  between  the  roots  of  the  two 
species  could  hardly  have  taken  place. 

The  specimen  of  Acacia,  which  was  about  a  meter  distant  from  the 
main  axis  of  the  cactus,  had  a  generalized  type  of  root-system.  It  con- 
sisted of  2  main  roots  and  about  12  smaller  ones.  Of  the  larger  roots,  one 
ran  north  of  the  main  axis  of  the  cactus  and  extended  beyond  the  east 
boundary  of  its  root-area,  as  fig.  12  indicates,  and  laj?^  from  9  cm.  to  40  cm., 
where  it  was  left,  beneath  the  surface.  The  other  roots  descended  rapidh- 
and  evidentl}^  penetrated  deep,  although  they  were  not  traced  more  than 
60  cm.  from  the  surface.  The  disparity  in  the  position  of  roots  of  O.  fulgida 
and  of  Acacia  was  so  great  that  there  could  be  no  active  competition 
between  the  two  species  for  ground  water. 

RiDDELLIA  COOPERI. 

Riddellia  coopcri  is  one  of  the  widely  distributed  shrubs  in  the  vicinity 
of  the  Desert  Laboratory,  occurring  not  only  on  Tumamoc  Hill  and  on  the 
bajada,  but  on  the  flood-plains  both  of  the  Santa  Cruz  river  and  of  West 
Wash.  It  was  studied  on  a  slope  separating  the  bajada  west  of  West  Wash 
from  the  flood-plain  of  the  wash  and  also  on  the  flood-plain  east  of  the  wash. 
The  soil  west  of  the  wash  is  red  clay  containing  coarse  particles  of  sand,  with 
the  caliche  hardpan  at  a  depth  of  55  cm. ;  the  soil  to  the  east  of  the  wash 
is  a  sandy  loam,  apparently  homogeneous  to  a  depth  exceeding  1.4  meters. 

Riddellia  is  about  30  cm.  high  and  consists  of  numerous  shoots,  the  ter- 
minal portion  only,  equal  to  about  half,  bearing  flowers,  and  the  proximal 
portion  only  bearing  leaves.  This  should  be  borne  in  mind  when  consider- 
ing the  stature  of  the  plant  as  presented  in  the  succeeding  measurements, 
since,  for  various  reasons,  the  measurements  were  made  to  include  both 
the  flowering  and  the  non-flowering  portions  of  the  shoots. 

The  specimens  situated  west  of  the  wash  had  a  generalized  type  of  root- 
system.  There  was  a  tap  root,  the  lower  portion  of  which  was  dead,  which 
gave  off  a  branch  that  descended  gradually  until  it  attained  a  depth  of  48 
cm.  where  its  tip  was  50  cm.  distant  from  a  line  dropped  straight  down 
from  the  crown  of  the  tap  root.  In  addition  to  this  root,  5  laterals  arose 
within  20  cm.  of  the  surface  of  the  ground  and  extended  in  a  fairly  hori- 
zontal direction  for  a  distance  of  i  meter  from  the  main  plant  axis.  These 
laterals  bore  few  branches  and  scarcely  any  filamentous  roots,  such  as  are 
a  feature  of  the  laterals  in  Franseria,  Encelia,  and  other  species,  and  the 
filamentous  roots  were  not  borne  in  groups,  but  occurred  singly. 

The  roots  of  Riddellia  extended  so  short  a  distance  in  a  lateral  direction 
that  the  only  root  found  within  its  root-area  was  a  single  one  of  Acacia 
constricta  (fig.   13). 

The  specimen  of  Riddellia  growing  east  of  the  wash  had  a  shoot  32  cm. 
high,  or  the  same  length  of  the  plant  just  described,  but,  as  will  appear 
directly,  in  several  particulars  its  root-system  was  unlike  that  of  the  other 


74 


ROOT    HABITS    OF    DESERT    PLANTS. 


Specimen.  A  tap  root  was  traced  down  40  cm.  and  gave  off,  within  15  cm. 
of  the  surface  of  the  ground,  6  main  laterals.  These  reached  out  less  than  i 
meter  from  the  main  root  axis,  and  certain  of  them  were  inclined  down- 
ward at  a  rather  sharp  angle.  The  laterals  gave  off  frequent  branches  of 
two  sorts,  filamentous  as  described  for  the  other  specimen,  and  relatively 
large  ones.  In  addition  to  the  main  laterals  and  their  branches,  there 
were  very  numerous  slender  roots  close  to  the  surface  and  taking  their 
origin  also  from  the  tap  root.  The  latter  type  of  roots  was  entirely  lacking 
in  the  first  plant  examined.  The  contrast  between  the  two  plants  in  this 
regard  is  shown,  but  not  adequately,  in  plate  19. 


Fig.  13. — Root-systems  of  Riddellia  cooperi. 
ij.   Horizontal  extension  of  root-system  which  was  growing  iu  the  sandy  loam  cast  of  West  Wash., 
showing  the  most  prominent  laterals  but  none  of  the  large  number  of  slender  ones  which  were 
at  base  of  shoot. 

b.  Vertical  extension  of  plant  shown  in  a.     Tap  root  penetrated  1.44  m.  beneath  surface. 

c.  Horizontal  extension  of  root-system  of  a  specimen  which  was  growing  west  of  the  wash  in  clay 

soil.     Dotted  line  shows  position  of  an  intruding  root  of  a  neighboring  Acacia  consiricia. 

In  considering  the  form  of  the  root-systems  of  the  two  plants,  both  hav- 
ing a  relatively  short  tap  root,  though  the  systems  were  such  as  to  lead  one 
to  expect  a  deeply  penetrating  root,  it  was  decided  to  excavate  the  roots  of 
another  plant  with  the  view  of  observing  especially  the  behavior  of  its  tap 
root.  Accordingly,  a  third  plant  was  selected,  growing  east  of  the  wash 
and  close  by  the  first  specimen  studied.  The  soil  conditions  where  the  sec- 
ond and  third  specimens  examined  were  growing  were  apparently  the  same. 
The  shoot  of  the  last  plant  studied  was  38  cm.  high.  The  root-system  of 
this  specimen  Avas  dominated  by  a  tap  root,  which  was  traced  to  its  end. 


ROOT-SVSTEMS   OF    PERKXXIALS. 


75 


1.44  meters  deep.  Like  the  root-systems  of  the  other  plants,  this  one  also 
comprised  several  laterals  which  were  borne  about  15  cm.  from  the  surface 
of  the  soil  and  which  reached  about  90  cm.,  but  then,  turning  sharply,  ran 
directly  downward  to  an  undetermined  depth.  The  slender  roots,  so 
numerous  on  the  last  plant  examined,  were  also  a  feature  of  this  one. 

Dasylirion  Texanum,  Yucca  Radiosa,  Yucca  sp.,   axd  Agave  sp. 

Up  to  this  point  the  plants  of  the  bajada,  which  have  been  described, 
grew  in  the  immediate  neighborhood  of  the  Desert  Laboratory,  or  at  most 
not  more  than  a  mile  distant,  but  the  plants  selected  for  the  purpose  of 
examining  types  other  than  had  been  seen  heretofore,  were  found  in  the 
region  west  of  the  Rincon  mountains,  about  20  miles  east  of  Tucson.  The 
bajada  at  that  point  was  perhaps  4  miles  from  the  base  of  the  main  range 
of  the  mountains  and  at  an  elevation  approximating  3,600  feet,  or  about 
1,300  feet  higher  than  Tucson.  The  greater  altitude  means  lower  tempera- 
tures and  greater  rainfall,  although,  as  there  are  no  records  of  the  locality, 
the  extent  of  the  difference  from  Tucson  in  these  regards  is  not  known. 

The  soil  conditions 
varied  where  the  plants 
studied  were  growing. 
At  the  Yucca  sp.  habitat 
the  upper  soil,  for  more 
than  a  meter,  was  sand 
and  red  clay  with  caliche 
underlying,  while  in  the 
habitat  of  Yucca  radiosa 
and  Dasylirion  texanum 
the  upper  soil,  although 
of  like  character,  was 
only  about  20  cm.  in 
thickness.  The  two 
plants  first  named  were 
50  meters  apart,  and 
were  about  200  meters 
from  Dasylirion.  The  Agave  sp.  was  found  on  the  upper  bajada  slope  near 
the  base  of  an  outlying  range  approximately  2,000  meters  south  of  the 
other  plants.  The  soil  was  similar  to  that  above  described,  except  that 
there  was  a  larger  amount  of  sand  and  rocks. 

In  the  immediate  vicinity  of  the  Agave  studied  were  observed  Calliandra 
sp.,  Encelia  jarinosa,  Foiiquieria  splendens,  Hyptis  sp.,  Opiintia  blakeana, 
and  Parkinsonia  micro pJiylla.  Of  the  two  specimens  of  Agave  examined, 
one  was  about  20  cm.  high  and  the  other  somewhat  larger.  The  roots  of 
young  agaves,  and  of  the  older  ones  as  well,  were  borne  in  a  cluster  at  the 
base  of  the  shoot.     All  of  the  roots  were  of  about  equal  length  and  very 


4. — a.  \'crtical  extension  of  root-system  of  'i'ucca 
radiosa  from  the  bajada  near  Rincon  mountains.  Dotted 
line  indicates  boundary  between  adobe  upper  soil  and 
underlying  rotten  caliche. 
Vertical  extension  of  root-system  of  Yucca  sp.  from  the 
bajada  near  the  Rincons. 


76  ROOT  HABITS  OF  DESERT  PLANTS. 

coarse.  The  older  ones  were  shriveled,  and  of  a  dark  red  color,  but  the  young 
roots,  which  were  put  out  from  the  base  of  the  plant  and  not  as  branches  of 
already  existing  roots,  were  stout  and  pinkish.  The  3'oung  roots  ended 
abruptly  and  very  evidently  functioned  more  as  storage  than  as  absorbing 
organs.     No  tap  root  was  seen  in  any  specimen  of  Agave  examined. 

The  bases  of  the  leaves  and  the  axis  of  the  stem  of  Agave  are  fleshy  and 
serve  the  purpose  of  storing  both  water  and  food -stuffs  against  the  flower- 
ing season  or  other  periods  of  need.  The  other  plant  studied  at  this  time, 
which  has  storage  facilities  above  the  ground,  Dasylirion  texanum,  has,  as 
will  appear  at  once,  a  similar  root-system. 

Turning  now  to  consider  Dasylirion  and  Yucca  and  the  habitats  in  which 
they  were  growing,  we  shall  find,  as  revealed  by  the  character  of  the  vege- 
tation as  well  as  shown  by  soil  differences,  that  the  bajada  farther  from  the 
mountain  base  does  not  have  uniform  conditions.  For  example,  in  the 
vicinity  of  Dasylirion  there  are  Acacia  constricta,  Bigelowia  hartwegii,  Opun- 
tia  blakeana,  Riddellia  coo  peri,  Yucca  sp.  Besides  these  species,  in  the 
vicinity  of  the  Yucca  sp.  studied,  the  following  occurred:  Parkinsonia 
micro phylla,  Parkinsonia  torreyana,  Prosopis  velutina,  and  Zizyphus  parryi. 
A.t  lower  altitudes,  as  at  Tucson,  these  species  do  not  occur  together,  but 
are  characteristic  of  the  different  w^ell-defined  physiographic  areas;  for 
example,  Parkinsonia  torreyana  grows  only  in  the  vicinity  of  washes,  or 
where  the  water  conditions  are  relatively  favorable,  ^vhile  P.  micro  phylla 
occurs  on  the  bajada  above  the  wash.  Both  Zizyphus  and  Prosopis  are 
chiefly  found  on  the  flood-plain,  occurring  not  at  all,  or  as  dwarfed  speci- 
mens only,  in  the  other  and  more  arid  areas. 

In  addition  to  the  differences  in  the  two  habitats  last  considered,  as 
revealed  by  the  difference  in  the  kinds  of  woody  plants  growing  in  them,  the 
Yucca  sp.  habitat  has  far  the  larger  population.  Without  further  investi- 
gation it  is  assumed  that  this  difference  is  w^holly  due  to  the  fact  that  the 
latter  habitat  has  deeper  soil,  and  hence  more  water  and  for  a  longer  time, 
than  the  alternate  one.  The  root-systems  of  the  plants  growing  in  the  first 
habitat,  Dasylirion  and  Yucca  radiosa,  will  be  first  described. 

The  specimen  of  Dasylirion  examined  was  a  meter  high.  The  roots 
were  exposed  on  the  southern  side  only  and  had  the  following  character- 
istics: A  large  number  of  roots,  each  about  5  mm.  in  diameter,  formed  a 
cluster  at  the  base  of  the  stem.  The  roots  ran  downward  at  an  acute  angle 
and  also  extended  out  into  the  soil  in  a  more  or  less  horizontal  direction. 
The  roots  were  all  coarse ;  the  most  shallow  were  1 5  cm.  beneath  the  surface 
and  the  deepest  about  36  cm.  deep.  The  longest  roots  were  the  most  super- 
ficial ones,  of  which  some  measured  2.25  meters,  though  they  were  mostly 
shorter  than  this  (plate  19c). 

Another  specimen  of  Dasylirion,  with  the  shoot  about  2  meters  high, 
possessed  a  root-system  essentially  like  the  smaller  plant  just  described, 
although  the  soil  at  the  place  was  somewhat  deeper  than  that  in  which  the 


ROOT-SYSTEMS   OF   PERENNIALS.  77 

smaller  plant  was  growing.  It  therefore  appears  that  the  root-system  of 
Dasylirion  is  essentially  a  comparatively  superficial  one. 

The  shoot  of  Dasylirion  functions  as  an  important  organ  for  food  and 
water  storage,  the  storage  apparatus  being  made  up  of  the  large  central 
axis  and  the  enlarged  leaf  bases.  The  entire  organ,  in  the  larger  plant 
studied,  was  25  X  30  cm.  In  another  specimen,  of  which  the  stem  axis 
was  2  meters  long,  the  water  capacity  would  be  very  considerable,  easily 
comparable  to  that  of  the  sahuaro. 

The  root-systems  of  two  specimens  of  Yucca  were  studied.  Both  of 
these  plants,  although  separated  by  some  little  distance,  were  growing 
under  practically  identical  soil  conditions.  The  old  specimens  of  Yucca,  of 
the  type  examined,  have  the  habit  of  growing  in  colonies,  to  the  exclusion 
of  plants  of  other  species,  but  the  younger  forms  occurred  singly. 

The  shoot  of  Yucca  sp.  was  70  cm.  high  and  bore  leaves  60  cm.  in  length. 
The  subterranean  portion  of  the  plant  consisted  of  a  branched  tuber-like 
root,  from  which  arose  several  roots  of  the  type  characteristic  of  Dasylirion 
and  Agave,  that  is,  coarse  and  little  branched.  The  tuber  measured  8  X 
25  cm.,  and  was  richly  supplied  with  sap.  The  branches  of  the  tuber  were 
also  fleshy,  and  three  or  four  of  them  bore  buds  destined  to  produce  shoots. 
From  the  tuber  26  roots  took  their  origin,  wholly  from  the  lower  surface, 
which  were  of  two  sorts,  namely,  immature,  which  did  not  function  as 
absorbing  organs,  and  mature  roots.  The  former  were  of  a  light  pink  color, 
1.5  cm.  in  diameter  and  about  8  cm.  in  length,  and  were  well  filled  with  sap. 
The  older  roots  were  dark  red  and  generally  shriveled ;  they  were  unbranched 
and  were  devoid  of  groups  of  filamentous  roots,  a  characteristic  of  many 
perennials  of  other  families  which  were  studied.  The  roots  were  variously 
placed.  The  one  penetrating  deepest  was  traced  to  its  end  1.23  meters 
beneath  the  surface.  Another,  1.22  meters  in  length,  ran  more  or  less  hori- 
zontally and  reached  8,  12,  and  22  cm.  beneath  the  surface  at  different 
points.  It  terminated  abruptly.  As  compared  with  the  root-system  of 
Dasylirion,  that  of  Yucca  sp.  can  be  said  to  be  poorly  developed. 

The  specimen  of  Yiicca  radiosa  studied  had  a  shoot  64  cm.  high  and  was 
situated  about  50  meters  from  the  other  species  of  Yucca  just  described. 
It  had  a  stout  main  root  from  which  arose  numerous  laterals.  The  main 
root  was  5  cm.  in  diameter  about  10  cm.  beneath  the  surface  of  the  ground 
and  expanded  rapidly  to  a  diameter  of  about  14  cm.,  which  it  maintained 
almost  to  its  end,  56  cm.  below.  The  tip  of  the  root  was  expanded  and 
divided  into  short  forks.  The  entire  root  was  thus  66  cm.  in  length  and, 
as  it  was  well  supplied  with  sap,  it  must  be  considered  an  important  water 
and  food  storing  organ  (plate  19,  d,  c). 

Laterals  were  given  off  from  the  main  root  from  a  point  about  20  cm. 
from  the  surface  to  the  end  of  the  root.  They  were  all  old  and  in  appear- 
ance were  like  those  of  the  other  species  of  Yucca.  As  in  the  other  species, 
they  were  comparatively  close  to  the  surface  of  the  ground. 


78  ROOT    HABITS    ()F    DESERT    PLANTS. 

THE   FLOOD-PLAIN. 

The  bottom  lands  which  come  into  this  study  comprise  not  only  the  flats 
near  West  Wash  and  those  along  the  Santa  Cruz  river,  but  also  a  branch 
of  the  latter,  the  mouth  of  a  wash  having  no  w^ell-defined  channel,  about 
9  miles  down  the  river,  west  from  Tucson.  The  first  two  physiographic 
areas  both  have  a  deep  surface  soil,  and  in  both  cases,  in  former  years 
before  the  Santa  Cruz  had  cut  itself  a  channel,  flood  waters  at  times  spread 
over  the  bottoms.  The  water  table  of  the  two  areas  is  at  different  depths : 
that  of  the  flood-plain  of  the  river  is  from  5  to  12  meters  from  the  surface, 
while  that  of  the  wash  is  much  deeper  and  probably  fluctuates  with  the 
seasons  or  the  years.  While  a  certain  amount  of  excavating  has  been  done, 
reliance  has  also  been  put  on  authenticated  reports  and  use  has  been  made 
of  natural  excavations,  as  the  caving  of  the  banks  of  the  river  and  the 
exposing  of  roots  by  other  severe  washouts.  On  the  river  flood-plain  have 
been  examined  the  rootsof  Koerbeyliniaspinosa,  Prosopis  vclutina,  and Zizy- 
pkus  parryi;  by  the  Wash  those  of  Pcnioccreus  greggii,  Ephedra  trifurca, 
and  Lycium  andersonii;  Opuntia  vivipara  was  studied  in  its  habitat  near 
the  Nine-mile  Water-hole.  The  root-systems  of  some  other  trees  or  shrubs, 
as  Olneya  tesota,  Parkinsonia  torreyana,  Condalia  spatlmlata  were  seen  in 
part  or  incidentally  in  connection  with  other  plants. 

THE   FLOOD-PLAIN   OF   THE   SANTA   CRUZ 
KcERBERLINIA  SpINOSA. 

Koerberlinia  is  one  of  the  most  striking  of  the  desert  plants  of  the  vicinity 
of  Tucson.  It  is  without  leaves  throughout  its  existence  and  the  branches 
are  reduced  to  rather  short,  stout  spines.  The  plants  are  usually  under 
a  meter  in  height  and  occur  in  colonies  which  are  frequently  circular  in 
form  and  4  meters  more  or  less  in  diameter.  Where  the  species  has  formed 
such  a  thicket  it  is  secure  from  injury  by  grazing  animals  and  constitutes  a 
safe  retreat  for  small  rodents.  The  distribution  of  the  plant  in  this  vicinity 
is  somewhat  limited.  It  does  not  occur  on  Tumamoc  Hill  or  by  West  Wash, 
but  on  the  flood-plain  of  the  Santa  Cruz,  by  the  edge  of  the  plain  as  well  as 
nearer  the  river,  and  on  the  bajada  where  soil  conditions  are  favorable. 

The  habitat  of  the  specimen  oi Kmberlinia  studied  is  on  the  line  between 
the  bajada  and  the  flood-plain  of  the  river  and  on  the  lower  slopes  of  the 
bajada  itself.  The  top  soil,  to  a  depth  of  30  cm.,  is  adobe  clay  with  sand 
or  gravel  admixture  and  under  this  is  the  caliche  hardpan.  On  the  flood- 
plain  just  below,  where  the  most  perfect  specimens  of  the  plant  were  grow- 
ing, the  soil  had  a  depth  characteristic  of  the  plain.  The  character  of  the 
specimens,  as  wxll  as  the  greater  depth  of  soil,  made  it  impracticable  to 
study  the  roots  of  the  better-developed  plants  of  the  flood-plain. 

The  root-systems  of  three  individuals,  or  groups,  were  examined  with 
the  purpose  of  learning  not  only  the  type  of  the  roots  but  also  the  manner 
of  development  of  the  colonial  character,  a  prevailing  habit  of  the  species. 


ROOT-SVSTEMS    OF    PERENNIALS.  79 

The  first  plant  studied  was  a  separate  individual  with  a  shoot  under  50 
cm.  in  height,  and  extremely  scraggy  and  thorny.  It  possessed  a  stout  tap 
root  which  went  down  75  cm.  and  then  turning  sharply  ran  for  an  unde- 
termined distance  horizontally.  At  its  crown  the  root  was  elliptical  in 
section  and  2  to  3  cm.  in  diameter.  It  gave  off  a  single  branch  about  12  cm. 
beneath  the  surface  of  the  ground,  and  a  single  adventitious  root  w^as  found 
which  took  its  origin  at  the  crown  of  the  tap  root.  This  root  was  5  mm. 
in  diameter  and  ran  straight  downward.  Both  the  tap  root  and  the  adven- 
titious root  penetrated  through  the  top  soil,  and  the  main  root  went  to  the 
caliche  before  turning  from  its  vertical  course.     (See  plate  20.} 

The  second  study  was  made  on  a  group  of  two  plants  of  unequal  size 
and  with  indications  that  they  had  had  a  common  origin.  The  tap  root 
of  the  larger  plant  went  straight  down,  after  the  manner  of  the  root  of  the 
plant  examined  earlier,  and  bore  two  main  laterals,  of  which  one  had  been 
partly  exposed  by  the  erosion  of  the  soil  surface,  and  one  lay  about  4  cm. 
beneath  the  surface.  The  latter  root  kept  its  distance  from  the  surface 
fairly  constant  for  the  distance  it  was  followed,  i  meter,  and  was  seen  to 
give  off  frequent  branches.  The  function  of  this  root  was  evidently  that  of 
absorption.  The  more  superficial  lateral  was  about  50  cm.  long  and  along 
its  upper  surface  bore  numerous  small  shoots,  and  on  its  lower  surface  man}^ 
short,  slender  rootlets.  No  deeply  penetrating  roots  were  found  beside 
the  tap  root  of  the  oldest,  really  parent,  plant.  It  appears,  therefore,  in 
these  two  plants,  that  the  younger  one  depended  on  the  parent  for  its  con- 
stant water  supply,  and  that  during  the  wet  seasons  this  could  be  supple- 
mented by  what  was  obtained  by  small  adventitious  rootlets  growing  along 
the  lower  surface  of  the  superficial  root. 

The  third  plant  group  studied  consisted  of  two  plants  of  unequal  size, 
but  both  relatively  small.  The  larger  of  the  plants,  the  parent  plant,  had 
a  pronounced  tap  root  which  gave  off  a  single  prominent  lateral  on  which 
was  borne,  15  cm.  from  its  place  of  origin,  the  daughter  plant.  Midway 
between  the  two  plants  the  connecting  root  was  observed  to  be  dead  and 
partly  decayed.  An  examination  of  the  daughter  plant  showed  that  it  had 
several  slender  and  short  absorptive  roots,  but  that  in  addition  there  was 
one  of  larger  diameter  which  ran  straight  down  and  functioned  as  a  tap 
root  (plate  20).  The  daughter  plant,  springing  as  a  shoot  from  the  sucker- 
like root  of  the  larger  form,  had  finally  become  independent  and  had  devel- 
oped a  deeply  penetrating  root-system  of  its  own. 

Other  plants  were  examined  in  which  the  daughter  shoots  were  large  and 
were  borne  on  stout  superficial  laterals  of  the  parent  plant  but  which  had 
not  developed  tap  roots.  In  the  largest  of  the  shoot-bearing  laterals  no 
absorptive  adventitious  roots  were  seen,  but  such  rootlets  are  present  in 
large  numbers  on  such  laterals  as  the  one  shown. 

It  would  appear  from  observations  on  the  distribution  and  root  habits 
of  Koerhcylinia ,  that  it  must  be  in  continuous  connection  with  a  water 
supply.     This  is  effected  either  by  each  plant  developing  a  tap  root  of  its 


8o  ROOT    HABITS    OF    DESERT    PLANTS. 

own,  or  by  maintaining  its  union  with  the  parent  and  deriving  the  needed 
supply  through  the  parent  root.  It  is  this  requirement  that  confines  the 
species,  apparently  of  the  extreme  xerophytic  type,  to  places  where  the 
soil  is  of  considerable  depth  and  the  water  relations  relatively  favorable. 

Prosopis  Velutina. 

Prosopis  shows  a  very  marked  power  of  accommodation  to  varied  con- 
ditions of  soil  and  water,  especially  the  former,  and  when  grown  under 
extreme  conditions,  exhibits  differences  in  growth  habit  so  great  as  nearly 
to  defy  recognition.  For  instance,  on  Tumamoc  Hill  and  on  the  bajada,  the 
species  occurring  to  a  limited  extent  in  both  places,  Prosopis  assumes  the 
form  of  an  irregular  bush,  but  on  the  flood-plain,  its  proper  habitat,  it  may 
become  a  tree  15  meters  or  more  in  height,  with  a  well-defined  bole.  Fre- 
quently, however,  along  the  river-bottoms  the  species  takes  the  form  of 
the  cultivated  eastern  apple.  By  an  earlier  obser\'er  (Havard,  The  Mezquit, 
American  Naturalist,  vol.  18,  page  450,  1884)  the  variation  in  the  size  of 
Prosopis  was  taken  as  an  index  of  the  depth  of  the  water  table :  if  the  tree 
was  large,  the  water  lay  close  to  the  surface ;  if  it  was  small,  the  water  table 
was  very  deep.  That  the  species  is  an  indication  of  the  presence  of  peren- 
nial water  is  said  to  be  the  belief  among  native  ranchers  of  southern  Arizona, 
who,  it  is  said,  may  even  follow  the  roots  of  mesquite  in  digging  wells — a 
"wa ter- witch  "  which  points  unerringh^  to  subterranean  water.  Although 
these  beliefs  are  largely  fanciful,  they  nevertheless  have  some  foundation ; 
where  the  mesquite  grows  large  the  perennial  supply  of  water  is  relatively 
close  to  the  surface,  and  where  it  is  small  the  water-supply  is  limited  but 
probably  confined  to  the  surface  water. 

How  deep  the  roots  of  Prosopis  may  penetrate  the  soil  is  difficult  to 
learn,  but  it  is  conditioned  on  the  character  of  the  soil,  the  depth  of  the  water 
table,  and  the  penetration  of  the  rains.  Where  a  substratum,  as  hard  caliche 
and  possibly  fine-grained  adobe  clay,  makes  difficult  or  prevents  deep  perco- 
lation of  flood  waters,  roots  of  plants  will  not  strike  deep ;  but  where  it  is 
such  as  to  permit  the  deep  sinking  of  the  rains,  or  the  rise  of  water  from 
the  perennial  water  suppl}^  the  plant  roots  may  also  penetrate  to  great 
depth.  The  most  deeply  placed  roots  of  Prosopis  known  to  the  writer  are 
those  of  plants  growing  by  the  Santa  Cruz,  which  penetrate  at  least  5  meters, 
but  I  have  been  informed  by  a  reliable  observer  that  the  roots  of  mesquite 
growing  by  a  tributary  of  the  Santa  Cruz  have  been  seen  to  reach  to  a  depth 
of  8  meters.  As  one  leaves  the  river  and  goes  toward  the  sides  of  the  flood- 
plain,  Prosopis  becomes  smaller  until,  at  the  edges  of  the  plain,  it  is  little 
more  than  a  large  bush.  The  water  table  also  is  deeper  at  the  sides  of  the 
flood-plain  than  near  the  river,  and  it  is  believed,  although  not  actually 
demonstrated,  that  the  roots  of  the  mesquite  reach  perennial  water  only 
where  the  water  table  is  relatively  close  to  the  surface. 

Where  the  surface  soils  do  not  permit  the  deep  penetration  of  the  roots  of 
Prosopis,  as  on  Tumamoc  Hill  and  the  bajada,  the  plant  derives  all  of  its 


Root  Habit  of  Koerberlinia  Spinosa. 

A.  Isolated  plant  with  prominent  tap-root. 

B.  Several  shoots  arising  from  a  sinj^le  sucker,  which  still  keeps  connection  with 
parent  tap-root. 

C.  Adventitious  absorption  roots  arising  from  a  sucker  which  has  not  formed  an 
independent  tap-root. 

1).  Secondary  formation  of  tap-root  in  a  group  with   connecting   sucker  root  no 
longer  living. 


a.  Peniocereus  greggii  from    flood-plain   near   West   Wash,  showing   fleshy  main 
root  and  most  important  laterals. 

b.  Condalia  spathulata  from  flood-plain  of  the  Santa  Cruz. 

c.  Root-system  of  Prosopis  velutina   partly  exposed   by   the  caving  bank   of  the 
Santa  Cruz.     Vertical  distance  from  surface  of  ground  to  water  is  approximately  5  m. 

d.  Young  plant  of  Prosopis.     The  squares  are  centimeters. 

e.  f,  g.  Cuttings  of  Opuntia  arbuscula  (Tucson),   O.  arbuscula  (Sacaton),  and   O. 
vivipara,  all  grown  under  similar  conditions  and   with   a  large  water  supply. 


ROOT-SYSTEMS   OF   PERENNIALS.  8 1 

water  supply  from  the  rains  directly,  and  also,  even  where  the  roots  may 
penetrate  deep  and  actually  do  so,  it  appears  from  the  abundance  of 
superficial  roots  that  the  species  growing  under  such  conditions  also  derive 
their  water  supply  mostly  from  surface  waters.  Thus,  although  it  is  char- 
acteristic of  the  young  plant  that  a  strong  tap  root  is  developed  (plate  2i), 
m  the  mature  form  many  of  the  laterals  remain  near  the  surface  of  the 
ground  and,  with  little  change  of  level,  may  run  1 5  meters  from  the  central 
axis  under  especially  favorable  conditions,  as  by  an  irrigating  ditch.  This 
observation  was  verbally  communicated  by  Prof.  R.  H.  Forbes,  of  the  Ari- 
zona Experiment  Station. 

CONDALIA   SpATHULATA   AND   ZlZYPHUS    PaRRYI. 

Condalia  and  Zizyphiis  are  both  dwellers  on  the  flood-plain  of  the  Santa 
Cruz  and  of  West  Wash,  but  the  characteristic  habitat  of  both  forms  is  on 
the  river  bottom.  In  this  vicinity  Zizyphus  reaches  its  largest  size  in  the 
forest  of  Prosopis  vclutina  near  the  old  San  Xavier  mission,  9  miles  south 
of  Tucson.  Both  species,  however,  were  studied  near  the  western  edge  of 
the  Santa  Cruz  flood-plain  about  6  miles  west  of  the  domain  of  the  Desert 
Laboratory. 

In  the  neighborhood  of  the  plants  examined  the  leading  perennials,  in 
addition  to  Condalia  and  Zizyphus  which  were  fairly  abundant,  were  Acacia 
constricia,  Bigclowia  hartwegii,  and  occasionally  Kcerberlinia  spinosa,  with 
many  arborescent  specimens  of  Prosopis  velidina. 

Soil  conditions  where  Condalia  and  Zizyphus  were  studied  are  character- 
istic of  the  edge  of  the  flood-plain,  that  is,  adobe  clay  with  particles  of  rock 
fragments  to  a  considerable  depth.  Condalia  is  evergreen,  but  Zizyphus  is 
deciduous,  dropping  its  leaves  soon  after  the  beginning  of  the  arid  autumn. 

The  specimen  of  Condalia  studied  was  74  cm.  in  height.  The  root-system 
of  the  plant  consisted  mainly  of  a  tap  root  with  numerous  small  and  a  few 
large  laterals.  The  tap  root  was  traced  directly  downward  1.3  meters  and 
where  left  it  was  8  mm.  in  diameter,  so  that  it  would  not  be  possible  to  state 
the  depth  attained,  but  without  question  it  was  much  more  than  actually 
demonstrated.  Atitscrown  the  tap  root  was  2.5  cm.  in  cross-section.  The 
leading  laterals  arose  as  follows:  three,  5  mm.  in  diameter,  originated  just 
below  the  surface  of  the  ground,  three  more  were  given  off  at  a  depth  of 
36  cm.,  and  one  arose  90  cm.  beneath  the  surface.  Throughout  the  first 
meter  of  the  tap  root  there  were,  between  the  larger  laterals,  very  many 
smaller  ones  about  i  mm.  in  diameter,  but  these  were  relatively  long.  The 
length  of  the  larger  laterals  was  not  learned.  Filamentous  roots  were  not 
seen  on  the  laterals. 

The  shoot  of  the  specimen  of  Zizyphus  examined  was  i  .4  meters  high  and 
bore  numerous  branches.  At  the  time  its  roots  were  examined  (April  22) 
it  was  not  in  leaf.  The  root-system  closely  resembled  that  of  Condalia; 
it  was  characterized  by  a  stout  tap  root  from  which  arose  several  laterals ; 
the  main  root  was  3.5  cm.  in  diameter  at  the  crown  and  was  traced  straight 


82  ROOT    HAI5ITS    OF    DESERT    PLANTS. 

down  to  its  end,  1.36  meters  deep;  18  laterals,  i  cm.  or  less  in  diameter, 
were  given  off  from  the  tap  root  within  15  cm.  of  the  surface  of  the  ground ; 
at  a  depth  of  i  meter  a  larger  lateral  was  given  off.  None  of  the  laterals 
bore  filamentous  roots  along  their  course,  as  did  those  of  Encelia,  Franscyia, 
and  other  plants  (plate  21/)). 

From  this  study  of  the  root-systems  of  Coiulalia  and  Zizypliiix  it  will  be 
seen  that,  more  than  in  any  other  plants  examined,  unless  perhaps  it  is 
Ka'berlima,  a  well-developed  tap  root  appears  to  be  an  essential  character, 
and  it  is  due  to  this  fact  that  these  species  are  confined  to  places  where  the 
soil  has  considerable  depth. 

Ephedra  Trikurca. 

Ephedra,  on  the  domain  of  the  Desert  I^aboratory,  occurs  exclusively  on 
the  flood-plain,  especially  that  by  West  Wash,  where  it  attains  large  size. 
It  is  there  associated  with  plants  characteristic  of  the  wash,  Acacia  greggii, 
Prosopis  vclnti)ia,  Parkinsonia  torrcyana,  and  a  few  specimens  of  Peniocereus 
greggii,  shown  in  ])late  21,  which  is  somewhat  rare  in  the  vicinitvof  Tucson. 


Fig.  5. — Root-s\steni  of  liplicdra  trijiirca  showing  position  of  tap  root  and  typical  laterals. 

The  Ephedra  whose  roots  were  examined  was  about  i  meter  high  and  the 
shoot  was  composed  of  numerous  slender  branches.  The  root-system  was 
characterized  by  a  stout  tap  root  and  few  relatively  slender  laterals.  The 
main  root  was  13  cm.  in  diameter  at  the  crown,  and  was  traced  75  cm.  to 
the  place  where  it  forked;  one  fork  was  followed  25  cm.  deeper,  a  total 
depth  exceeding  i  meter,  and  the  end  was  not  found.  The  laterals  were  of 
two  sorts,  a  larger  and  a  smaller  kind.  The  latter  were  confined  to  the 
crown  and  resembled  the  slender  roots  in  an  analogous  situation  in  the  plants 
of  Franseria,  growing  by  the  Wash.  The  larger  laterals  originated  from 
15  to  50  cm.  beneath  the  surface  of  the  ground,  and,  as  is  usual  with  the 
roots  of  plants  which  grow  where  the  soil  is  deep,  either  went  straight 
down  or  fairly  horizontal!}-.  The  depths  and  the  lengths  of  two  of  the  lat- 
erals may  be  taken  as  being  representative  of  all  of  the  rest ;  one  arose  1 5 
cm.  from  the  surface  and  kept  this  distance  very  closel}-  to  end,  1.8  meters 
from  the  main  root ;  the  other  lateral,  which  arose  at  about  the  same  depth, 
after  running  straight  out  10  cm.,  turned  downward  at  a  sharp  angle  to  a 
depth  of  57  cm.,  when  it  took  a  horizontal  course  for  a  short  distance. 


ROOT-SYSTEMS    OF    PERENNIALS. 


Opuntia  Vivipara. 


In  the  vicinity  of  Tucson  the  cacti  usually  occupy  the  more  arid  situations 
as  the  bajada  or  Tumamoc  Hill,  and  avoid  the  flood-plain,  but  among  the 
exceptions  to  this  condition  should  be  included  Pcnioceyeus  greggii  and 
Opuntia  vivipara.  The  former,  owing  to  the  fleshy  main  root,  requires  con- 
siderable earth-room  for  its  full  development,  and  usually  occurs 
where  the  soil  is  relatively  deep.  Opuntia  vivipara is'k.nown  only 
from  a  locality  not  far  from  the 
mouth  of  a  wash  which  debouches  on 
the  flood-plain  of  the  vSanta  Cruz  near 
the  Nine-mile  Water-hole. 

Opuntia  vivipara  is  of  the  cylind- 
rical type  and  is  remarkable  especially  for  the  great 
number  of  young  plants  which  take  their  origin  from 
fallen  "joints"  and  which  cover  the  ground,  beneath 
every  large  individual  of  the  species,  with  a  fairly 
dense  growth.  The  manner  of  origin  of  most  of  the 
plants  is  indicated  by  the  figure  in  plate  21,  which 
also  shows  something  of  the  fleshy  nature  of  the 
young  roots. 

The  root-systems  of  several  plants  were  examined 
and  what  follows  is  merely  a  resume  of  the  observa- 
tions made  on  them.  In  most  regards  the  root- 
system  of  the  mature  plant  of  vivipara  is  similar  to 
other  arborescent  opuntias,  e.g.,  that  of  O.  versicolor, 
as  described  in  a  foregoing  paragraph ;  that  is,  there 
is  an  anchoring  system  and  an  absorbing  system,  both  well  differ- 
entiated. But  in  a  certain  particular  the  roots  of  this  species  are  unlike 
those  of  any  other  cactus  examined :  they  are  usually  slender,  but  occasion- 
ally one  is  found  which  is  fleshy,  although  the  more  distal  portion  as  well 
as  the  proximal  portion  may  be  of  the  usual  type.  The  departure  from 
the  usual  root-type  Avill  appear  in  the  following  measurements.  A  lateral 
of  the  usual  form,  that  is,  3  to  4  mm.  in  diameter  up  to  a  point  50  cm.  from 
its  place  of  origin,  may  then  become  1.5  cm.  in  diameter  and  retain  this 
larger  size  for  25  cm.,  while  nearer  as  well  as  farther  from  the  main  root  the 
lateral  will  be  of  the  slender  type.  The  peculiarit>'  of  the  root-system  of 
the  species,  in  addition  to  the  fact  that  fleshy  roots  are  present,  is  that  both 
slender  and  fleshy  roots  occur  on  the  same  plant. 

It  was  shown  in  the  case  of  Opuntia  arhuscula  that  the  fleshy  roots  of  that 
species  gave  rise  to  plants  as  a  regular  habit,  and  it  was  learned  that  the 
fleshy  roots  of  O.  vivipara  are  capable  of  doing  the  same  thing,  although 
the  characteristic  has  not  been  observed  in  nature. 


Fig.  16. — Horizontal  exten- 
sion of  root-system  of 
Opuntia  vivipara.  All 
laterals  in  this  specimen 
were  slender. 


EXPERIMENTAL  CULTURES. 

Although  the  primary  object  of  the  present  research  was  to  learn  the 
most  striking  facts  regarding  the  root-system  of  mature  desert  plants,  as 
a  necessary  preparation  to  later  experimental  work,  problems  arose  dur- 
ing the  course  of  the  study  on  which  experiments  were  carried  out  intended 
to  be  suggestive  rather  than  necessarily  conclusive.  The  experiments 
naturally  centered  around  the  relation  of  the  roots  to  water  supply:  (i) 
as  regards  formation  of  adventitious  and  temporary  rootlets;  (2)  the  direc- 
tion of  growth  or  position  of  the  roots;  (3)  as  regards  the  quality  of  fleshi- 
ness in  the  young  as  well  as  mature  roots  of  certain  opuntias. 

The  root-systems  of  many  species  of  perennials  and  a  few  annuals  are 
provided  with  filamentous  roots  in  groups  of  about  6  each.  In  perennials 
these  roots  appear  during  favoring  seasons  and  disappear  when  such  sea- 
sons have  passed.  In  annuals  their  behavior  is  not  so  clear  but  their  pres- 
ence is  probably  also  associated  with  an  increase  of  the  water  supply.  The 
leading  experiments  undertaken  along  this  line  may  be  briefly  stated. 
Among  the  perennials,  Franseria  deltoidea  and  Encelia  farinosa  form  tempo- 
rary rootlets  in  midwinter,  if  the  ground  is  moistened,  but  other  species 
such  as  Covillea  tridentata,  Fouqmena  splendens,  Lycium  andersonii,  and 
Opuntia  discata,  are  not  provided  at  this  season  with  newly  developed 
rootlets.  Therefore,  in  certain  of  the  species  named,  besides  an  improve- 
ment of  the  water  relations,  a  condition  has  to  be  fulfilled  before  the  tempo- 
rary absorbing  roots  may  be  formed,  and  this  very  clearly  is  higher  tem- 
perature, since  freshly  formed  rootlets  are  to  be  found  on  these  plants  in 
early  autumn.  However,  such  a  plant  as  Opuntia  discata  can  absorb  water 
in  the  winter  season  without  the  development  of  recognizably  new  rootlets, 
as  a  series  of  experiments  in  the  winter  season  well  shows. 

On  November  11,  1908,  after  several  weeks  of  drought,  a  small  specimen 
of  Opuntia  discata  was  well  watered,  and  the  watering  was  repeated  the 
following  day.  Six  measurements  on  the  thickness  of  the  flat  joints  were 
made  on  as  many  different  places;  31  hours  after  the  water  was  applied 
the  joints  had  begun  to  increase  in  thickness;  the  increase  in  thickness 
continued  for  three  days,  after  which  the  maximum  diameter  was  main- 
tained for  an  unknown  period.  On  the  sixth  day  after  irrigating,  roots 
of  the  plant  were  carefully  removed  from  the  soil  and  examined.  No  newly 
formed  rootlets  were  present.  In  summer,  however,  such  roots  are  formed 
on  this  species,  and  the  vegetative  activityof  the  plant  is  very  great,  as  shown 
by  new  growth,  the  formation  of  leaves,  and  a  high  rate  of  transpiration. 
Among  the  annuals  the  behavior  of  the  roots  as  regards  the  formation 
of  adventitious  rootlets  also  appears  to  be  varied,  although  probably  con- 
stant for  any  species.     A  large  proportion  of  the  annuals  studied  were  seen 


a.  Cultures  of  Fouquieria  splendens  and  of  garden  watermelon  showing  differenc 
in  character  and  development  of  extreme  types  of  root-systems. 

b.  Shoot  of  Opuntia  vivipara  springing  from  a  fleshy  root;  natural  size. 


EXPERIMENTAL   CULTURES. 


85 


w 


to  possess  them,  but  they  were  not  to  be  found  in  a  few,  and  in  one  species 
their  formation  could  not  be  induced  experimentally.  In  Amsinckia  spec- 
tabilis,  these  rootlets  arc  present  as  rudiments,  whatever  may  have  been 
the  water  conditions,  but  are  only  brought  to  full  development  under  favor- 
able moisture  relations;  while  in  Rafinesquia,  as  stated  above,  no  adventi- 
tious rootlets  were  seen  in  nature  and  none  could  be  induced  to  form  in 
experiments.  The  cultures  were  carried  on  in  the  plant  house  where  the 
temperatures  were  higher  than  out  of  doors.     (See  page  40). 

In  December-February,  1907-8,  a  culture  was  set  for  the  purpose  of  seeing 
the  relative  extent  of  the  roots  of  a  typical  desert  seedling  and  those  of 
a  pronounced  mesophyte.  The  seeds  planted  were  of  the  garden  water- 
melon and  of  Fouquicria  spJendcns.     When  the  plants  w^ere  taken  up  the 

shoots  of  each  species  were  about  of  a 
height,  although  the  transpiring  sur- 
face of  the  water-melon,  naturally, 
was  much  greater.  The  root-system 
of  the  seedlings  of  the  two  species  was 
found  to  be  very  unlike;  that  of  the 
water-melon  was  three  to  four  times 
as  long  as  the  shoot  and  bore  very 
many  laterals  of  the  first  order,  while 
that  of  the  Fouquicria  was  only  about 
as  long  as  the  shoot  and  bore  few  lat- 
erals. The  relative  development  of 
the  two  is  imperfectly  shown  in  plate 
22.  The  behavior  of  the  roots  of  Fou- 
quieria  brought  about  the  starting  of 
for  the  purpose  of 
learning  the  direction  which  the  tap 
root  of  a  typical  desert  plant  would  take  if  provided  with  an  abundance  of 
water.  Water  poles  of  porous  clay  tubes  were  placed  in  opposite  ends  of 
a  box  and,  after  the  soil  moisture  coming  from  the  poles  had  reached  an 
equilibrum  throughout  the  box,  seeds  of  water-melon  and  of  Parkinsonia 
aculeata  were  sown  at  different  distances  from  the  centers  of  water  supply 
(fig.  17).  The  root  of  the  water-melon  went  straight  down,  while  that  of 
Parkinsonia  inclined  away  from  the  water  pole  in  a  marked  manner.  The 
difference  in  reaction  of  the  roots  of  the  two  plants  is  thus  a  striking  one, 
whether  the  immediate  cause  is  too  much  water  or  too  little  oxygen,  or 
other  causes  not  now  suspected. 

This  form  of  culture  was  later  repeated  using  bulbs  of  Brodicea  capitata 
in  place  of  the  other  plants  previously  used,  with  the  distribution  of  water 
from  porous  clay  cups,  of  the  kind  employed  by  Livingston,  arranged  as 
before,  but  with  fine  and  coarse  earth  arranged  in  alternating  vertical  col- 
umns.    Bulbs  were  placed  both  in  the  fine  soil  and  in  the  coarse,  and  on 


Fig.    17. — Culture    of    water-melon     (1)     and 

Fouquicria  splcndetis  (2jin  a  soil  kept  con-    aUOthcr     CUltUrC 
stantly  moist  by  a  porous  clay  cup  shown 
at  left. 


86  ROOT    HAHITS    OF    DESERT    PLANTS. 

the  line  separating  the  two.     The  results  of  this  culture  have  already  been 
given  above  and  need  only  be  summarized  here. 

The  plant  on  the  adobe-clay  side  of  the  culture  had  developed  two  sorts 
of  roots :  a  fleshy  tap  root  which  incUned  away  from  the  water  pole  and 
ended  in  a  curl,  and  several  of  the  usual  sort  (plate  23).  The  bulb  had 
entirely  disappeared.  The  plant  on  the  sand  side  of  the  box  (both  were 
very  close  to  the  opposite  kind  of  soil)  had  developed  two  kinds  of  roots 
also,  a  large  tap  root  and  a  few  laterals.  The  tap  root  went  straight  down 
without  regard  to  the  water  pole  (plate  23).  In  the  latter  plant  the  bulb 
had  not  wholly  disappeared. 

In  connection  with  the  behavior  of  the  Brodioea,  whatever  may  be  the 
immediate  underlying  causes,  it  is  of  interest  to  review  the  reaction  of 
certain  cacti  to  analogous  natural  and  cultural  conditions.  In  the  course  of 
observations  on  the  roots  of  the  cacti  it  was  learned  that  frequently  the 
seedlings  of  the  arborescent  forms  were  provided  with  fleshy  roots,  and  also 
that  in  the  root-systems  of  the  mature  plants  of  Opmitia  vivipara  some 
of  the  roots  were  slender  and  some  fleshy,  and  also  that  all  of  the  laterals 
of  Opuntia  arhuscida  were  flesh3\  In  the  latter  species  the  description 
applies  only  to  the  plants  from  near  Tucson;  those  seen  near  Sacaton 
are  slender  throughout.  Cultures  of  cuttings  from  the  Sacaton  form  of 
arbuscula,  the  Tucson  representative  of  the  same  species,  and  of  Opuntia 
vivipara,  were  grown  in  the  experiment  house  during  the  spring  and  sum- 
mer. The  soil  in  which  the  plants  were  placed  was  sand  and  adobe,  about 
equal  parts.  The  culture  was  frequently  watered  throughout  the  entire 
period.  On  November  2 1  the  plants  were  carefully  removed  from  the  soil 
and  it  was  noted  that  all  of  the  larger  roots  of  O.  vivipara  were  fleshy;  the 
longer  roots  of  the  arbuscula  from  Sacaton,  the  species  which  does  not  pos 
sess  fleshy  roots  in  its  natural  surroundings,  were  fleshy  also,  as  were  those 
of  the  specimens  from  Tucson.  But  the  fleshiness  of  the  last  was  the  least 
marked  of  any  (plate  21).  It  would  appear,  therefore,  that  the  character 
of  fleshiness  in  these  cacti  and  in  Brodicpa  is  to  be  associated  with  an 
abundant  water  supply. 

Besides  serving  as  water-storage  organs  the  fleshy  roots  of  Opuntia 
arbuscula,  from  Tucson,  also  propagate  the  species,  as  mentioned  above 
and  described  by  Preston  (Non-sexual  propagation  of  Opuntia,  Bot.  Gaz., 
p.  128,  vol.  31,  1 901).  This  appears  to  be  a  very  common  way  of  increas- 
ing the  number  of  individuals.  To  learn  whether  similar  conditions 
might  obtain  in  those  roots  of  0.  vivipara  which  were  fleshy,  one  was  brought 
to  the  experimental  plantation  and  given  water  freely  for  some  months. 
From  this  root  a  single  well-developed  shoot  with  roots  appeared  and  con- 
tinued to  grow  vigorously  (plate  21). 


PHYSIOLOGICAL.  FEATURES  OF  ROOT-SYSTEMS. 
"CHARACTER"  IN  ROOTS. 

The  roots  of  desert  plants,  possibly  to  a  greater  extent  than  the  roots 
of  plants  of  the  more  humid  regions,  are  remarkable  for  their  individuality. 
The  roots  of  each  genus,  often  perhaps  of  each  species,  possess  peculiarities 
of  form,  of  branching  habits,  of  color,  of  texture,  of  position  in  the  ground, 
or  of  more  subtle  physiological  reactions.  In  how  far  these  features  can 
be  used  for  systematic  purposes  is  yet  to  be  shown,  but  there  is  no  question 
of  their  importance  in  the  field  of  plant  biology,  particularly  in  ecology. 

It  has  been  found  convenient  to  group  the  types  of  root-systems  into 
generalized  and  specialized  forms  with  a  physiological  rather  than  a  sys- 
tematic bearing.  By  a  generalized  root-system  is  meant  one  that  has  both 
the  tap  root  and  the  laterals  well  developed.  Such  roots  penetrate  the 
ground  deeply  and  reach  out  widely.  To  this  type  belong  most  of  the 
desert  perennials,  of  which  Franscria  deltoidca  and  Prosopis  velutina,  among 
the  shrubs  and  trees,  may  be  taken  as  representative.  Nearly  all  the 
annuals,  also,  belong  to  this  type.  There  are  two  forms  of  specialized 
roots,  those  with  the  tap  root  the  chief  feature,  as  in  Ephedra  trifurca  and 
Koerbcrlinia  spinosa,  and  those  with  the  laterals,  which  are  placed  near  the 
surface  of  the  ground,  especially  well  developed.  Of  the  latter,  the  cacti 
are  almost  the  sole  representatives.  So  far  as  desert  plants  are  con- 
cerned, it  is  probably  true  that  generalization  in  root-systems  looks  toward 
mesophily,  and  specialization  toward  xerophily. 

The  specialized  root-systems  of  either  form  are  changed  little  with  envi- 
ronment, but  the  generalized  roots  a  e  often  extremely  variable,  ranging 
from  a  pronounced  tap  root  to  a  marked  development  of  the  laterals,  depend- 
ent on  soil  characters  and  water  relations.  Thus  it  will  appear  clear  that 
rigidity  or  plasticity  of  root-system  may  be  an  important  factor  in  the 
local  distribution  of  a  species,  a  feature  to  be  discussed  below. 

One  of  the  most  striking  characters  of  the  root-systems  of  perennials  is 
the  variation  in  the  branching  habit,  although  as  the  habit  is  a  matter  of 
degree  rather  than  of  kind  it  is  not  possible  to  express  the  difference  in 
exact  terms.  Under  parallel  conditions,  the  roots  of  any  species  act  con- 
sistently, and  where  the  conditions  are  much  changed,  as  from  the  bajada 
to  the  flood-plain,  and  modification  of  the  branching  habit  results,  still  the 
induced  variation  is  superimposed  on  the  specific  and  familiar  habit  in  such 
a  way  that  the  proper  habit  is  easily  recognized.  The  roots  of  Covillea 
branch  repeatedly,  wherever  the  plant  is  growing,  but  those  of  Fouquieria 
are  little  branched.  The  most  richly  branched  root-system  observed  was 
of  Opuntia  arbuscula,  which  covered  the  ground  so  completely  that  it  would 

87 


88  ROOT  HABITS  OP  DESERT  PLANTS. 

have  been  impossible  for  any  other  plant,  perennial  or  annual,  to  gain  a 
foothold  without  encountering  the  roots  of  this  species.  Krameria  can- 
escens,  the  parasite,  however,  has  a  root-system  that  is  scarcely  branched 
beyond  the  first  order.  Among  the  annuals  it  was  observed  that  the  root- 
system  of  those  appearing  in  summer  was  more  richly  branched  than  the 
roots  of  the  winter  forms,  and  that  the  annuals  both  of  summer  and  of 
winter  varied  greatly,  but  consistently  in  the  species,  so  that  genera  at  least 
could  be  distinguished  merely  by  the  branching  habits  of  the  roots. 

The  more  superficial  laterals  of  the  generalized  root-systems  of  perennials 
and  a  few  roots  of  the  cactus  type  of  the  specialized  root-systems  are  pro- 
vided with  filamentous  roots,  borne  in  groups,  of  limited  period  of  activity. 
These  temporary  absorption  roots,  referred  to  in  this  paper  as  deciduous 
roots,  are  formed  always  during  the  rainy  season,  either  of  summer  or 
winter,  and  persist  until  the  soil  about  them  becomes  unbearably  dry. 
The  length  of  this  period  is  not  known,  but  probably  is  not  far  from  the 
time  which  limits  the  growth  of  annuals,  from  three  to  six  weeks,  or  even 
more.  Similar  roots  are  to  be  seen  in  some  of  the  annuals,  particularly  in 
Amsinckia  among  those  of  winter,  and  Trianthema  of  those  of  summer,  but 
are  regularly  absent  from  certain  other  species. 

THE  RELATION  OF  ROOTS  TO  SOIL  TEMPERATURE. 

The  relation  of  the  roots  of  annuals  and  of  perennials  to  the  temperature 
of  the  soil  can  be  presented  briefly.  As  appeared  in  an  earlier  section  of 
this  paper,  the  grand  course  of  heat  movement  finds  its  maximum  just 
prior  to  the  summer  rains,  and  its  minimum  in  late  winter  or  early  spring. 
With  the  coming  of  the  rains  in  summer  the  soil  temperature  falls  immedi- 
ately and  considerably,  but  the  rains  of  winter  do  not  appear  to  have  so 
marked  effect  on  the  temperatures  of  the  soil.  However,  when  more  com- 
plete temperature  observations  are  at  hand,  the  winter  rains  may  be  found 
to  have  a  marked  and  important  effect,  particularly  in  the  way  of  altering 
the  temperature  of  the  uppermost  soil  levels. 

From  what  is  known  regarding  the  temperature  of  the  soil  at  a  depth  of 
15  cm.,  it  appears  that  the  summer  annuals  are  not  subjected  to  very  intense 
heat,  in  fact  not  greatly  exceeding  80°  F.,  but  those  of  winter  may  experi- 
ence temperatures  very  close  to  the  freezing  point.  At  a  depth  of  30  cm. 
the  soil  is  not  as  cold  as  at  less  distance  from  the  surface,  but  the  minimum 
is  reached  later  in  the  season  than  at  the  higher  level,  so  that  the  roots  of  the 
winter  annuals  may  be  subjected  to  the  following  conditions :  During  the 
warmer  davs  of  winter  and  early  spring  the  superficial  soil  may  attain  a 
temperature  relatively  high,  while  the  deeper  soil  is  yet  cold.  The  condi- 
tions for  most  favorable  water  absorption  are  not  present  in  winter,  there- 
fore, and  the  effect  is  a  limitation  of  the  development  both  of  root  and  of 
shoot.  Reversed  conditions  are  to  be  found  in  summer,  when  the  lower 
soil  lavers  at  the  time  of  the  rains  and  the  appearance  of  the  annuals  are 


Brodiaea  capitata  grown  in  adobe  clay,  and  b,  in  sand,  in  plant-house  cultures 
Main  root  of  clay-grown  plant  curved  away  from  source  of  water  supply. 


PHYSIOLOGICAL   FEATURES    OF    ROOT-SYSTEMS.  89 

warmer  than  the  upper  soil  layers.  The  growth  conditions  both  of  shoot 
and  of  root  in  summer,  therefore,  are  most  favorable,  with  the  result  that 
the  summer  annuals  are  very  luxuriant,  the  shoots  carrying  a  large  leaf 
surface  and  the  roots  being  especially  well  developed. 

The  relations  of  the  roots  of  perennials  to  soil  temperatures  are  at  once 
similar  and  unlike  the  relations  of  the  annuals.  They  are  similar  in  that 
the  deciduous  roots,  which  are  developed  on  the  more  superficial  laterals, 
occupy  the  same  soil  horizon  as  the  roots  of  the  annuals,  are  developed  at 
a  time  when  the  annuals  appear,  and  die  with  their  passing.  The  persistent 
portions  of  the  roots  of  the  perennials  are  subject  to  the  greatest  tempera- 
ture extremes  of  the  horizon  they  occupy,  which  in  the  most  superficial 
roots,  like  certain  of  the  cacti,  must  be  very  great,  although  the  exact 
measure  is  not  known.  The  peripheral  absorption  roots  of  perennials, 
both  of  the  specialized  and  the  generalized  types  of  root-s^^stems,  have 
still  other  temperature  relations.  It  is  probable  that  these  roots  in  the 
generalized  root-systems,  and  in  the  Ephedra  type  of  the  specialized  roots, 
remain  active  throughout  the  year,  but  that  in  the  cactus  type  of  the 
specialized  roots,  especially  in  such  forms  as  Opuntia  arhuscula  and  Echino- 
cactus  -wislizeni,  they  are  formed  at  the  time  that  the  deciduous  roots  appear 
and  die  when  the  latter  kind  of  roots  die.  The  temperature  relations  of 
these  roots  are  thus  seen  to  be  extremely  complicated. 

Most,  perhaps  all,  of  the  perennials  are  active  during  the  summer  rainy 
season,  but  many  do  not  renew  growth  or  come  into  flower  in  winter.  This 
is  clearly  a  temperature  relation  and  finds  its  reaction  in  the  formation 
of  deciduous  roots,  and  probably  in  other  ways,  in  the  root-svstems.  It 
has  been  found  impossible,  for  example,  to  induce  the  formation  of  decidu- 
ous roots  in  Opuntia  discata  in  winter,  and  deciduous  roots  have  not  been 
seen  in  Fouquieria  splendens  at  this  season,  though  when  the  examination 
of  the  roots  of  Fouquieria  was  made,  temporary  roots  were  seen  in  Encelia 
farinosa,  Eycium  andersonii,  and  other  plants. 

THE  RELATION  OF  ROOTS  TO  WATER. 

The  water  relations  of  the  plants  in  the  vicinity  of  the  Desert  Labora- 
tory are  very  complex,  owing  partly  to  diversity  in  the  plants  themselves 
and  partly  to  essential  differences  as  regards  soil  water  in  their  habitats. 
These  differences  lie  chiefly  in  the  origin  of  the  water  supply  as  well  as  in 
the  length  of  time  which  the  water  of  the  soil  is  available  for  use.  On 
Tumamoc  Hill  the  entire  supply  is  derived  directly  from  the  rains ;  on  the 
bajada  the  water  comes  in  the  main  directly  from  the  rains,  but  some  is 
also  received  as  superficial  run-off  or  by  seepage  from  Tumamoc  Hill  or 
other  higher  ground ;  on  the  flood-plains,  in  addition  to  the  rains  and  the 
water  which  comes  from  Tumamoc  Hill  and  other  higher  ground,  there  is  a 
water-table  of  unknown  depth  by  West  Wash,  varying  from  5  to  1 2  meters 
beneath  the  surface  on  the  plain  by  the  Santa  Cruz.  The  water-table  by  the 


90  ROOT   HABITS    OF    DESERT    PLANTS. 

Santa  Cruz  is  sufficiently  near  the  surface  to  permit  its  being  reached  by 
the  roots  of  the  larger  plants,  as  Prosopis  vclutma.  It  has  been  learned* 
that  the  Santa  Cruz  is  at  present  lower  than  the  water-table,  so  that  the 
river  acts  as  a  ditch.  In  times  not  very  remote  there  was  no  well-defined 
channel  of  the  river,  as  at  present,  but  the  water  spread  over  the  plain, 
which  might  indicate  that  formerly  the  water-table  was  nearer  the  surface 
than  it  is  now  and  thus  more  easily  reached  by  the  roots  of  plants.  The 
changes  in  the  position  of  the  water-table  have  probably  operated  to  modify 
profoundly  the  flora  of  this  habitat. 

Not  only  do  the  sources  of  water  vary  in  the  different  habitats,  but  also 
the  periods  during  which  it  is  available  for  the  use  of  plants.  Tumamoc 
Hill  and  the  bajada,  partly  for  reasons  of  differences  in  water  supply,  are 
the  most  arid,  and  the  flood-plain  of  the  vSanta  Cruz  the  least  so.  The 
character  of  the  soil  and  its  depth  are  also  important  factors  in  this  con- 
nection. The  soil  of  the  bajada  and  Tumamoc  Hill  is  usually  less  than  50 
cm.  deep,  while  the  soil  by  West  Wash  is  over  2  meters  and  that  of  the  flood- 
plain  of  the  Santa  Cruz  is  5  meters,  more  or  less.  Thus  the  arid  condi- 
tions of  the  bajada  and  Tumamoc  Hill,  with  the  least  water  supply,  are 
greatly  increased  by  the  slight  soil  covering.  The  length  of  time  elapsing 
after  a  through  wetting  before  the  soil  of  the  bajada,  at  a  depth  of  15  cm., 
becomes  air-dry,  is  about  three  weeks,  but  moisture  at  this  depth  on  the  flood- 
plain  of  the  Santa  Cruz  remains  for  six  weeks.  In  this  instance,  however, 
in  addition  to  the  fact  that  more  water  is  present  in  the  habitat,  the  char- 
acter of  the  soil  as  regards  its  fineness  is  an  important  factor. 

Annuals  are  directly  affected  by  the  rains  of  the  particular  habitats 
where  they  are  placed,  while  the  perennials  are  in  part  directly  and  in  part 
indirectly  dependent  on  obtaining  water  by  this  means.  On  the  bajada 
and  on  Tumamoc  Hill,  water  is  mostly  derived  directly  from  the  rains,  and 
only  such  perennials  as  are  provided  with  water-storage  organs,  or  are  the 
most  resistant,  can  live  or  attain  the  best  development  in  these  habitats. 

The  depth  to  which  the  roots  of  annuals  penetrate  the  ground  is  directly 
controlled  bv  the  depth  of  the  penetration  of  the  rains  of  the  season  and 
the  persistence  of  the  annuals  is  mainly  directly  dependent  on  the  length 
of  time  the  water  remains  in  the  soil  where  they  are  growing.  As  above 
stated,  within  a  period  of  three  weeks  following  the  rains,  the  superficial 
soil  on  the  bajada,  to  a  depth  of  15  cm.,  may  become  air-dry,  while  that 
of  Tumamoc  Hill  and  of  the  flood-plain  may  retain  moisture  for  a  period 
exceeding  six  weeks.  Since  the  largest  part  of  the  roots  of  most  annuals  do 
not  reach  deeper  than  20  cm.  it  is  seen  that  the  period  of  their  activity  is 
definitely  fixed  and  comparatively  brief. 

Annuals  having  the  most  deeply  penetrating  root-systems,  and  which 
are  provided  with  checks  against  excessive  water  loss,  survive  longest  after 
the  wet  season  has  passed.     KaUstrcemia  grandiflora,  for  example,  a  sum- 

*Communicated  to  the  writer  by  Prof.  G.  E.  P.  Smith,  of  the  Arizona  Experiment 
Station. 


PHVSIOLOGICAU    FEATURES    OF    ROOT-SYSTEMS.  ()l 

mer  annual,  outlives  many  other  forms,  and  should  additional  rains  occur 
somewhat  out  of  season,  but  before  the  plant  has  succumbed,  it  will  revive 
and  accompHsh  an  astonishing  extent  of  growth.  Among  the  winter 
annuals,  such  deeply  rooted  forms  as  Amsinckia  spcctabilis  and  Phacdia 
tanacetifolia  will  survive  long  after  Bowlcsia  lobata,  or  other  plants  with 
shallow  roots,  have  perished. 

How  closely  the  character  of  the  water  supply,  the  amount  of  water, 
and  the  progressive  drying  out  of  the  soil,  are  related  to  the  character  of 
the  root-system  of  annuals,  does  not  appear  from  the  observations  here 
reported.  In  certain  instances,  however,  the  root-system  is  greatly  modi- 
fied by  the  water  relations.  For  example,  it  has  been  repeatedly  seen  that 
the  longest  and  frequently  the  greatest  number  of  laterals  arise  within  4  to  5 
cm.  of  the  surface  of  the  soil.  Without  definite  knowledge  of  the  water 
movements  at  that  depth,  it  may  be  assumed  that  during  the  period 
of  greatest  growth  the  soil  at  this  depth  has  an  adequate  water  supply 
and  provides  practically  all  of  the  water  necessary  for  the  plant,  and  that  it 
is  only  later,  when  by  progressive  drying  out  the  upper  soil  becomes  too 
dry  for  benefiting  the  plant,  that  it  obtains  its  water  at  the  greatest  depth 
and  mainly  by  means  of  the  elongated  tap  root.  In  certain  cases,  as  in 
Amsinckia,  where  precociously  formed  root  rudiments  are  present,  the 
coming  of  unseasonal  rains  induces  growth,  and  the  superficial  roots  are 
again  in  condition  to  absorb  water. 

The  reaction  of  the  root-systems  of  the  perennials  is  much  more  complex 
than  that  of  the  annuals,  inasmuch  as  a  portion  of  the  roots  live  through- 
out the  year  and  a  portion  endure  during  favorable  seasons  only.  The  situ- 
ation is  further  complicated  by  the  fact  that  each  habitat  differs  from  every 
other  habitat  as  regards  water  relations.  The  great  difference  in  the  char- 
acter of  the  root-systems  of  the  perennials  and  their  possible  variability 
under  different  conditions  have  also  to  be  considered. 

The  peripheral  rootlets  of  the  generalized  root-systems  are  probably 
living  throughout  the  year,  since  they  are  in  soil  which,  even  in  dry  seasons, 
contains  sufficient  moisture  for  absorption,  but  the  deciduous  rootlets,  both 
of  the  generalized  and  the  specialized  root-systems,  are  present  and  func- 
tional only  during  the  more  favorable  seasons, -chiefly  in  summer.  The 
peripheral  roots  of  the  specialized  forms  of  the  cactus  type,  however,  like 
the  deciduous  rootlets,  are  present  only  during  the  more  favorable  seasons. 
From  these  circumstances  it  is  seen  that  the  active  period  of  absorption  of 
plants  with  the  latter  type  of  roots  is  a  very  restricted  one,  while  the  absorp- 
tion period  of  the  generalized  root-system  and  of  the  Ephedra  type  of  the 
specialized  system,  although  most  active  during  the  existence  of  the  decid- 
uous rootlets,  is  continuous  from  season  to  season. 

In  plants  having  generalized  root-systems,  it  is  probable  that  the  pene- 
tration of  the  roots,  the  character  of  the  soil  permitting,  is  equal  to  the 
penetration  of  the  rains.     On  the  bajada  this  is  limited  by  the  presence  of 


92  ROOT   HABITS    OI?   DESERT   PLANTS. 

the  hard  pan  which  occurs  within  50  cm.,  or  less,  of  the  surface,  and  on 
Tumamoc  Hill  by  the  underlying  rock.  On  the  flood-plain,  where  the  soil  is 
sand  and  adobe,  as  by  West  Wash,  the  penetration  is  probably  about  2 
meters,  but  on  the  Santa  Cruz  flood-plain  water  from  the  rains  is  thought 
not  to  penetrate  the  ground  over  i  meter,  although  it  probably  goes  below 
that  depth  in  places. 

The  penetration  of  the  roots  of  the  cactus  type  apparently  presents 
another  problem,  since  certain  of  the  cactus  TOot-systems,  Opuntiaarbuscula 
particularly,  lie  within  2  to  5  cm.  of  the  surface  or  well  within  the  depth 
attained  by  water  either  during  summer,  when  the  most  active  growth 
occurs,  or  in  winter.  The  relation  in  this  instance  may  be  with  a  proper 
air  supply,  rather  than  with  the  penetration  of  the  rains  alone. 

Where  there  is  considerable  depth  of  soil  and  the  water  table  is  close  to 
the  surface,  as  in  places  near  the  Santa  Cruz  river,  the  conditions  are  most 
favorable  for  the  deep  penetration  of  roots.  Under  such  conditions  the 
water  chain  during  the  rainy  season  may  be  continuous  from  the  water 
table  to  the  surface  and  so  favor  deep  penetration.  It  is  here  that  Prosopis 
vehUina  and  Populus  sp.  may  be  in  position  to  obtain  perennial  water  and 
attain  a  large  growth.  The  roots  of  Prosopis  have  here  been  seen  to  reach 
5  to  8  meters  below  the  surface,  which  is  apparently  the  greatest  depth 
reached  bv  any  plant  in  this  vicinity. 

RELATIONS  OF  THE  ROOTS  OF  NEIGHBORING  PLANTS. 

The  desert  plants  vary  greatly  in  their  abundance.  It  is  well  known 
that  shrubs  inhabiting  the  bajada  are  comparatively  remote  from  one 
another.  On  the  flood-plain  the  same  species  may  form  a  fairly  dense 
thicket.  In  favorable  seasons  the  annuals,  both  of  the  bajada  and  the 
flood-plains,  are  occasionally  so  numerous  as  to  completely  hide  the  ground. 
It  is  evident,  therefore,  that  the  roots  of  the  different  classes  of  plants, 
annuals  or  perennials,  hold  var3'ing  relations  to  each  other,  which  are  dif- 
ferent in  accordance  with  the  kind  of  habitat  they  occupy. 

The  mutual  relations  of  the  roots  of  annuals  and  of  perennials  vary  also 
wdth  the  character  of  the  perennial  roots,  but  only  as  the  roots  of  annuals 
find  sufficient  room  for  full  development  in  all  of  the  habitats. 

On  the  bajada  and  Tumamoc  Hill  the  roots  of  such  annuals  as  Anoda 
thurheri,  Kallstroeniia  grandiflora,  Solanum  elcBagnifolium,  Erodium  cicu- 
tarium,  and  others  which  have  a  deeply  penetrating  root-system,  may 
reach  as  deep  as  the  roots  of  the  perennials  having  a  generalized  type  of 
root-system.  But  on  the  flood-plain  the  roots  of  the  two  classes  of  plants 
do  not  occupy  the  same  horizon  due  in  part  to  the  fact  that  the  roots  of  the 
annuals  differ  little  if  any  in  habit  from  those  on  the  bajada,  but  mainly 
because  of  the  deep  penetration  of  the  perennials.  This  can  be  illustrated 
by  a  single  example.  The  roots  of  Covillea  on  the  bajada  may  extend 
away  from  the  main  axis  as  far  as  4  meters,  while  on  the  flood-plain  the 


PHYSIOLOGICAL   FEATURES    OF   ROOT-SYSTEMS.  93 

lateral  extent  may  be  less  than  2  meters.  In  the  latter  case  there  is  the 
additional  fact  that  the  root-system  of  Covillea  is,  as  a  whole,  deeper  than 
on  the  bajada.  Competition  between  annuals  and  perennials  on  the  flood- 
plain,  therefore,  may  be  considered  a  negligible  quantity,  but  competition 
between  the  two  classes  of  plants  on  the  bajada  during  the  vegetative 
seasons  is  undoubtedly  very  keen,  and  the  presence  of  annuals  on  the 
bajada  is  probably  an  important  factor  in  bringing  about  the  sparse  char- 
acter of  the  perennial  flora  of  the  habitat. 

Since  the  roots  of  the  specialized  type  of  the  Ephedra  form  penetrate 
deep  and  reach  laterally  to  but  a  small  extent,  the  roots  of  the  annuals  do  not 
come  into  close  relation  wuth  them,  but  the  relation  of  the  roots  of  annuals 
and  of  the  specialized  roots  of  the  cactus  type  is  peculiar  in  that  usually 
the  roots  of  such  perennials  are  more  shallowl}^  placed  than  those  of  the 
annuals.  Owing,  however,  to  the  fact  that  in  the  root-system  of  annuals, 
laterals  are  ordinarily  prominent  4  to  5  centimeters  beneath  the  surface, 
during  the  rainy  season,  the  roots  of  the  two  classes  occupy  the  same  horizon 
and  undoubtedly  enter  into  keen  competition  for  water.  Soon  after  the  close 
of  the  rainy  period  the  superficial  roots  of  the  annuals  and  the  deciduous 
rootlets  of  the  perennials,  owing  to  the  progressive  desiccation  of  the  soil, 
can  no  longer  remain  functional.  With  the  drying  of  the  surface  layers  the 
more  superficial  roots  of  the  plants  having  the  generalized  root  type  also 
cease  absorbing,  and  the  entire  amount  taken  in  by  the  root-system  of 
such  plants  comes  from  the  deeper  levels.  At  this  period  the  most  deeply 
placed  roots  of  such  annuals  as  mentioned  above  are  placed  in  close  rela- 
tion with  the  deeper  absorbing  roots  of  the  perennials  with  generalized 
roots,  and  the  competition  is  transferred  from  the  more  superficial  to  the 
more  deeply  placed  roots  both  of  annuals  and  of  perennials.  Finally  the 
annuals  mature,  their  roots  absorb  in  decreasing  amounts,  and  competi- 
tion with  the  roots  of  other  plants  ceases. 

Where  the  growth  of  annuals  is  most  dense,  the  roots  intermingle  and 
occupy  the  soil  fairly  completely.  This,  however,  occurs  only  in  the  case 
of  perennials  when,  as  on  the  bajada,  plants  of  one  species,  or  plants  having 
similar  types  of  root-systems,  are  growing  in  proximity.  The  most  strik- 
ing instance  observed  of  the  encroaching  of  the  roots  of  one  species  on  the 
territory  occupied  by  the  roots  of  another  plant  of  the  same  species  was 
that  of  Covillea,  where  60  roots  of  neighboring  Covilleas  were  seen  either  in 
physical  contact  or  lying  very  near  the  roots  of  the  plant  being  specially 
studied. 

The  roots  of  plants  with  specialized  root-systems  of  the  cactus  type  do 
not  occupy  the  same  horizon  as  the  roots  of  perennials  having  the  general- 
ized forms  of  roots,  and  therefore  do  not  compete  with  them  for  water. 
This,  however,  is  true  only  of  such  cacti  as  have  the  most  superficial  roots ; 
those  like  Opuntia  fulgida,  with  a  root-system  which  approaches  the  gen- 
eralized type  in  character,  probably  approach  this  type  in  their  mode  of 
distribution  in  the  soil,  particularly  where  the  soil  is  shallow. 


94  ROOT   HABITS    OF    DESERT    PLANTS. 

ROOT  HABITS  AND  PLANT  DISTRIBUTION. 

The  character  of  the  root-system  of  desert  plants  as  an  ecological  factor 
is  often  of  great  importance.  The  direction  taken  by  the  roots  as  a  factor 
in  the  causes  underlying  the  distribution  of  plants  may  be  illustrated  in  a 
simple  manner,  thus :  Let  a,  b,  and  c  represent  the  three  leading  root  types, 
namely,  those  having  both  laterals  and  tap  root  well  developed,  generalized 
root-system  (a),  those  with  the  tap  root  as  the  most  prominent  feature  (b), 
and  those  with  the  laterals  of  special  prominence  (c).  Of  plants  having  a, 
b,  or  c  forms  of  roots,  other  conditions  including  temperature  relations 
being  equal,  a  will  have  the  most  general  distribution,  b  will  have  the  most 
limited  distribution,  while  plants  with  c  roots  are  in  a  measure  intermediate 
in  this  regard.  In  plants  with  the  last  type  of  roots,  however,  it  is  probable 
that  factors  other  than  the  root  character  exercise  the  controlling  influence. 

Of  plants  with  generalized  root-systems  Acacia,  Celtis,  Encelia,  Lycium, 
Parkinsonian  and  Prosopis  may  be  found  to  some  extent  in  each  habitat, 
both  where  the  soil  is  deep  and  where  it  is  shallow.  With  these  forms 
should  also  be  included  the  annuals  except  those  with  bulbous  roots. 
Plants  with  this  root  type  are  either  evergreen  or  deciduous,  but  all  have  a 
fairly  large  transpiring  surface,  and  in  certain  of  them,  especially  Encelia, 
the  leaves  are  of  good  size. 

Plants  having  a  prominent  tap  root,  the  b  type,  include  Cotidalia,  Penio- 
cereus  greggii.  Ephedra,  Kcrrberlinia,  and  Zizyphus.  These  forms  are  re- 
stricted to  localities,  preferably  the  flood-plain,  where  the  soil  has  sufficient 
depth  for  the  development  of  the  main  root. 

The  cacti  are  nearly  all  provided  with  the  c  form  of  roots,  but  to  them 
should  be  added  Jatropha  and  Kranieria,  the  latter  a  parasite.  The  cacti 
occur  both  on  the  bajada  and  on  Tumamoc  Hill,  and  not  on  the  flood-plain 
of  the  Santa  Cruz  or  by  West  Wash  to  any  extent.  They  are  the  plants 
par  excellence  of  the  bajada. 

Summing  up  the  relation  between  the  type  of  root-system  and  the  dis- 
tribution of  plants,  according  to  the  notation  given  above,  we  have  the 
following :  Plants  with  a  type  of  roots  may  be  found  on  the  bajada,  Tuma- 
moc Hill,  and  on  the  flood-plains  of  West  Wash  and  of  the  Santa  Cruz. 
Plants  having  the  b  type  of  roots  occur  only  on  the  flood-plains  of  the  wash 
and  the  river.  Plants  with  the  c  type  are  chiefly  on  the  bajada,  although 
they  are  common  also  on  Tumamoc  Hill,  to  which  certain  of  them,  notably 
Carnegiea,  are,  for  the  most  part,  confined. 

The  different  root  types  are  differently  related  to  the  distribution  of  the 
plants.  The  generalized  roots  (a  type)  facilitate  distribution  because  of 
their  plasticity,  while  the  specialized  roots  of  the  b  type  restrict  the  distri- 
bution because  of  their  rigiditv  and  the  need  of  considerable  depth  of  soil. 


SUMMARY. 

1 .  On  Tumamoc  Hill  there  is  sufficient  moisture  in  the  soil  at  a  depth  of 
30  to  40  cm.  to  be  available  to  plants  all  of  the  year.  The  soil  of  Tumamoc 
Hill  and  the  flood -plain  of  the  vSanta  Cruz  at  a  depth  of  15  cm.  may  be  air- 
dry  within  six  weeks  following  rains  and  that  of  the  bajada  within  three 
weeks. 

2.  The  highest  soil  temperatures  are  reached  in  July  just  preceding  the 
summer  rains;  the  lowest  temperatures  of  the  soil,  at  a  depth  of  15  cm., 
are  reached  in  midwinter  and  a  depth  of  30  cm.  in  late  winter. 

3.  The  roots  of  most  annuals  do  not  penetrate  the  soil  deeper  than  20 
cm.  The  largest  development  of  laterals  of  annuals  takes  place  4  to  5  cm. 
from  the  surface  of  the  ground.  Rudimentary  roots  of  the  first  order 
were  seen  in  Aiiisinckia,  Erodium,  Eritrichium,  Harpagonella,  Malva,  and 
Pectocarya  among  the  winter  annuals,  and  Amaranthus,  Boerhaavia,  Clado- 
tlirix,  and  Triantlicma  of  the  summer  forms.  These  rudiments  are  further 
developed  only  under  favorable  moisture  conditions,  such  as  out-of-season 
rains,  when  they  become  of  great  importance  to  the  plant. 

4.  The  root-systems  of  the  summer  annuals,  which  are  distinct  from  the 
winter  annuals,  are  especialh'  well  developed.  This  characteristic  is  due 
to  the  great  luxuriance  of  shoots  of  the  summer  forms,  owing  to  the  favor- 
able vegetative  conditions  of  this  season,  not  least  of  which  is  the  warm  soil. 
In  winter  the  soil  at  the  depth  attained  by  the  roots  of  the  annuals  is 
colder  than  the  superficial  la3'ers  and  does  not  present  the  most  favorable 
conditions  for  water  absorption.  In  winter,  also,  the  shoot  development 
of  annuals  is  less  than  in  summer. 

5.  Perennials  have  three  t3'pcs  of  roots,  namely,  the  generalized  type, 
with  the  tap  root  and  the  laterals  both  well  developed,  and  two  specialized 
forms,  of  which  one  type  has  a  prominent  tap  root  and  the  other  promi- 
nent laterals.  Covillea  tridcntata  and  Prosopis  vehitina  are  representatives 
of  plants  with  generalized  roots,  while  Koerberlinia  spinosa  and  a  few  other 
plants  have  specialized  roots  of  the  first  type,  and  most  of  the  cacti  have 
specialized  roots  of  the  second  kind. 

6.  The  roots  of  perennials  growing  on  Tumamoc  Hill  and  on  the  bajada 
do  not  penetrate  as  a  rule  deeper  than  30  cm.,  the  depth  of  the  available 
soil ;  while  those  on  the  flood-plain  of  West  Wash  attain  a  depth  of  2  meters, 
and  those  by  the  Santa  Cruz  may  reach  as  deep  as  5  meters  or  more. 

7.  The  most  shallow  rooted  perennials  observed  were  Opuntia  arbuscula 
(whose  roots  frequently  do  not  lie  more  than  2  cm.  below  the  surface) 
and  EchinocciCtus  wislizcni.  The  arborescent  opuntias  have  roots  which 
approach  the  generalized  type. 


96  SUMMARY. 

8.  Most  cacti  have  two  divisions  of  the  root-system :  an  anchoring  and 
an  absorbing  system.  In  Carnegiea  gigantea  the  anchoring  roots,  in  old 
plants,  are  assisted  in  their  mechanical  function  by  the  enlarged  bases 
of  the  laterals.  This  to  a  certain  extent  is  true  of  the  arborescent  cacti 
as  well,  but  the  lack  of  secondary  development  of  this  character  in  Echino- 
cactus  wislizeni  is  the  chief  reason  why  plants  in  age  lean  sharply,  or  fall. 

9.  Fleshiness  in  the  roots  of  the  opuntias  is  a  condition  resulting  directly 
from  an  abundant  supply  of  water.  Whether  some  species  exhibit  greater 
tendency  toward  fleshiness  than  others  was  not  determined.  A  similar 
reaction  was  seen  in  Brodicea  capitata. 

10.  The  most  superficial  roots  of  the  generalized  type  of  root-systems 
and  a  few  of  the  specialized  type  of  the  cactus-form  bear,  on  the  larger  lat- 
erals, filamentous  roots  in  groups  of  6  or  more.  These  are  formed  during 
the  rainy  season,  especially  in  the  summer,  and  die  with  the  drying  out 
of  the  soil.  The  deciduous  rootlets  are  of  great  advantage  to  the  plants 
in  that  they  greatly  and  quickly  increase  the  absorption  surface,  without 
at  the  same  time  increasing  the  distance  of  water  transport — a  factor  of 
undoubted  importance  where  the  transpiration  rate  is  high. 

1 1 .  Competition  as  evidenced  by  the  relationship  of  the  roots  of  neigh- 
boring plants  may  be  summarized  as  follows :  The  roots  of  annuals  inter- 
mingle and  often  occupy  the  same  horizon.  Where  perennials  of  a  single 
species  occur  together  on  the  bajada,  the  roots  of  one  plant  ma}^  reach  to 
and  intrude  upon  the  root-area  of  its  neighbor.  Thus  in  studying  Covillea 
on  the  bajada,  60  roots  of  neighboring  Covilleas  were  encountered  which 
either  were  in  contact  with  the  roots  of  the  plant  studied  or  were  in  the 
same  horizon.  On  the  flood-plain  competition  among  the  roots  may  not 
be  so  keen  as  on  the  bajada.  The  roots  of  annuals  growing  on  the  bajada 
reach  as  deep  as  most  perennials  in  the  same  habitat,  and  since  they 
occur  in  large  numbers  competition  with  them  must  be  an  important 
causal  factor  contributing  to  the  sparseness  of  the  perennial  vegetation  of 
the  bajada.  The  annuals  also  come  into  competition  with  the  shallow 
rooted  perennials  through  the  laterals  which  are  developed  on  the  tap  root 
of  the  annuals  4  to  5  cm.  beneath  the  surface  of  the  ground. 

12.  Perennials  with  the  generalized  type  of  root-system  have  the  widest 
local  distribution,  and  those  with  a  pronounced  development  of  the  tap  root 
have  the  most  limited  distribution.  Plants  with  laterals  well  developed, 
the  cacti  especially,  are  most  abundant  on  the  bajada  and  on  Tumamoc 
Hill,  where  the  soil  is  shallow,  and  seldom  occur  on  the  flood-plains. 


ROOTH, 


Can>//i 


